Remove hoedown from rustdoc

Is it really time? Have our months, no, *years* of suffering come to an end? Are we finally able to cast off the pall of Hoedown? The weight which has dragged us down for so long?

-----

So, timeline for those who need to catch up:

* Way back in December 2016, [we decided we wanted to switch out the markdown renderer](#38400). However, this was put on hold because the build system at the time made it difficult to pull in dependencies from crates.io.
* A few months later, in March 2017, [the first PR was done, to switch out the renderers entirely](#40338). The PR itself was fraught with CI and build system issues, but eventually landed.
* However, not all was well in the Rustdoc world. During the PR and shortly after, we noticed [some differences in the way the two parsers handled some things](#40912), and some of these differences were major enough to break the docs for some crates.
* A couple weeks afterward, [Hoedown was put back in](#41290), at this point just to catch tests that Pulldown was "spuriously" running. This would at least provide some warning about spurious tests, rather than just breaking spontaneously.
* However, the problems had created enough noise by this point that just a few days after that, [Hoedown was switched back to the default](#41431) while we came up with a solution for properly warning about the differences.
* That solution came a few weeks later, [as a series of warnings when the HTML emitted by the two parsers was semantically different](#41991). But that came at a cost, as now rustdoc needed proc-macro support (the new crate needed some custom derives farther down its dependency tree), and the build system was not equipped to handle it at the time. It was worked on for three months as the issue stumped more and more people.
  * In that time, [bootstrap was completely reworked](#43059) to change how it ordered compilation, and [the method by which it built rustdoc would change](#43482), as well. This allowed it to only be built after stage1, when proc-macros would be available, allowing the "rendering differences" PR to finally land.
  * The warnings were not perfect, and revealed a few [spurious](#44368) [differences](#45421) between how we handled the renderers.
  * Once these were handled, [we flipped the switch to turn on the "rendering difference" warnings all the time](#45324), in October 2017. This began the "warning cycle" for this change, and landed in stable in 1.23, on 2018-01-04.
  * Once those warnings hit stable, and after a couple weeks of seeing whether we would get any more reports than what we got from sitting on nightly/beta, [we switched the renderers](#47398), making Pulldown the default but still offering the option to use Hoedown.

And that brings us to the present. We haven't received more new issues from this in the meantime, and the "switch by default" is now on beta. Our reasoning is that, at this point, anyone who would have been affected by this has run into it already.

Author: GuillaumeGomez

.gitmodules

@@ -2,10 +2,6 @@
 	path = src/llvm
 	url = https://github.com/rust-lang/llvm.git
 	branch = master
-[submodule "src/rt/hoedown"]
-	path = src/rt/hoedown
-	url = https://github.com/rust-lang/hoedown.git
-	branch = rust-2015-09-21-do-not-delete
 [submodule "src/jemalloc"]
 	path = src/jemalloc
 	url = https://github.com/rust-lang/jemalloc.git

COPYRIGHT

@@ -192,28 +192,6 @@ their own copyright notices and license terms:
     USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
     OF SUCH DAMAGE.
 
-* Hoedown, the markdown parser, under src/rt/hoedown, is
-  licensed as follows.
-
-    Copyright (c) 2008, Natacha Porté
-    Copyright (c) 2011, Vicent Martí
-    Copyright (c) 2013, Devin Torres and the Hoedown authors
-
-    Permission to use, copy, modify, and distribute this
-    software for any purpose with or without fee is hereby
-    granted, provided that the above copyright notice and
-    this permission notice appear in all copies.
-
-    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR
-    DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE
-    INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
-    FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
-    SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR
-    ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA
-    OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
-    OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
-    CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
-
 * libbacktrace, under src/libbacktrace:
 
     Copyright (C) 2012-2014 Free Software Foundation, Inc.