checkdoc: Delete redundant check for wide docstrings

checkdoc: Delete redundant check for wide docstrings

With Emacs 28.1, wide docstrings are now flagged by the byte-compiler.
The logic in the byte-compiler for catching these issues is complex
and continuously evolving, particularly to avoid incorrectly flagging
`substitute-command-keys` substitutions.  Unfortunately, the checkdoc
implementation incorrectly flags correct docstrings as overly wide.

Rather than duplicating the byte-compiler logic in checkdoc, which
would introduce unnecessary maintenance overhead, we acknowledge that
this check is now redundant and remove it.  Users utilizing the
byte-compiler, whether manually or through `flymake`, will continue to
be warned about this issue.

* lisp/emacs-lisp/checkdoc.el (checkdoc-this-string-valid-engine):
Remove check for overly wide docstrings; duplicates byte-compiler
logic.

(cherry picked from commit 408ad273eeadf72dea11b89ea2a44f36ea0e2295)