Avoid infinite questions if TAGS file was removed

Avoid infinite questions if TAGS file was removed

When the visited TAGS file was removed, commands that depend on
TAGS, such as auto-completion, may invoke an interactive question
whether a user wants to re-read the file.  From that point on,
the question will be asked over and over, because the file no
longer exists, which results in mtime mismatch and inability to
"fix the mismatch" by reading from the file.  Fix that by simply
ignoring the mismatch if the file no longer exists.
* lisp/progmodes/etags.el (tags-verify-table): Avoid infinite
questions if TAGS file was removed.  (Bug#75946)

(cherry picked from commit eb12b6d153b8ad3f9e3fe23761a98021d8071293)