Remove buggy regexp use in comint-output-filter

Remove buggy regexp use in comint-output-filter

This reverts commit 2012-07-02T16:18:02!monnier@iro.umontreal.ca
which perhaps could be rethought and recommitted, and perhaps
it’s no longer needed now that comint-use-prompt-regexp
is almost always nil.
* lisp/comint.el (comint-output-filter): Don’t try to skip
repeated prompts, since comint-prompt-regexp typically begins
with "^" and the resulting "^^" in the regular expression does
not have the desired effect.  Noted by Mattias Engdegård in:
https://lists.gnu.org/r/emacs-devel/2019-03/msg00380.html