Remove obsolete doc references to XEmacs (Bug#37899)

Remove obsolete doc references to XEmacs (Bug#37899)

* doc/misc/edt.texi (Changes, Goals):
* doc/misc/emacs-mime.texi (Charset Translation):
* doc/misc/gnus-faq.texi (FAQ 4-15):
* doc/misc/pcl-cvs.texi (Bugs):
* doc/misc/srecode.texi (SRecode Minor Mode):
* lisp/cedet/srecode/insert.el (srecode-insert-ask-variable-method):
* lisp/follow.el:
* lisp/leim/quail/ipa.el (ipa-x-sampa, ipa-kirshenbaum):
* lisp/net/newsticker.el:
* lisp/obsolete/iswitchb.el:
* lisp/progmodes/hideshow.el:
* lisp/savehist.el (savehist-file):
* lisp/vc/ediff.el: Remove obsolete references to XEmacs in doc
strings and comments.