Remove fn pretty_printing_compatibility_hack

[petrochenkov]

#73345 adds a hack to support some older versions of proc-macro-hack and procedural-masquerade which included incorrect asserts.
The hack detects some specific enums used by those crates literally by their names and names of their variants.

Using newer versions of those crates fixes the issues

• proc-macro-hack requires a major version bump so it doesn't happen automatically
• for procedural-masquerade only a bugfix version update is required, so cargo update should do the job, but the old version is fixed in hundreds of Cargo.locks according to crater.

This hack needs to be removed eventually.
Now it's July 1 2020, so on July 1 2021 or later we need to run crater once more to estimate the use of these old versions again.
Then we could assume that crates not updating from outdated versions for so long are not maintained and we can break them and remove the hack.

[JohnTitor]

@petrochenkov It's now July 30th, could we start a crater run?

[petrochenkov]

IIRC, @Aaron1011 recently did some further deprecation work related to these hacks, but I'm not sure what is the current status exactly.

[Aaron1011]

I did a Crater run for this fairly recently, which found 135 regressions: #85387

[nnethercote]

Note: once nt_pretty_printing_compatibility_hack is removed, then Group::flatten can be removed, along with tokenstream::Cursor::append.

[tjallingt]

The current PR trying to remove the hack is #93275.
With #94063 as an alternative that keeps the hack for the rental crate.