Co-authored-by: Eric Huss <eric@huss.org>

This removes some content for const_refs_to_static which seems to
duplicate content elsewhere. In particular:

* Remove the examples of allowed use of `static` in `const_eval.md`. We
  don't show examples for any of the other permitted uses. Additionally,
  I believe the other points in the list already cover concerns such as
  interior mutability (and possibly more precise way, since the existing
  content mentions things like transient places).
* I think a single statement that reads of `extern` statics is
  sufficient to follow the pattern of all the other allowed behaviors.
* Removed the examples from the `constant-items.md` chapter about
  allowing use of `static`s. Again, we generally don't duplicate what
  is allowed unless there is some explicit statement indicating that
  it is not. Also, I am reluctant to duplicate what is allowed between
  `const_eval.md` and `constant-items.md`.

Additionally, this drops the statement that all access is "safe" since
there isn't anything that would imply that it is unsafe.