Remove many items obsolete since Emacs 23.2 and 23.3

Remove many items obsolete since Emacs 23.2 and 23.3

* lisp/allout.el (allout-init):
* lisp/emacs-lisp/shadow.el (shadows-compare-text-p):
* lisp/ffap.el (ffap-version):
* lisp/filecache.el (file-cache-choose-completion):
* lisp/help.el (print-help-return-message):
* lisp/image-mode.el (image-mode-maybe):
* lisp/imenu.el (imenu-example--name-and-position):
* lisp/international/mule-cmds.el (princ-list):
* lisp/mail/rmail.el (rmail-highlight-face):
* lisp/minibuffer.el (read-file-name-predicate):
* lisp/mouse.el (mouse-choose-completion):
* lisp/progmodes/cc-cmds.el (c-forward-into-nomenclature):
* lisp/progmodes/xscheme.el
(advertised-xscheme-send-previous-expression):
* lisp/simple.el (completion-base-size)
(choose-completion-delete-max-match, exchange-dot-and-mark):
* lisp/subr.el (eval-next-after-load):
* lisp/term.el (term-dynamic-simple-complete):
Remove items, obsolete since Emacs 23.2 and 23.3.
* doc/misc/cc-mode.texi (Movement Commands): Doc fix.
* doc/lispref/help.texi (Accessing Documentation):
* lisp/emacs-lisp/edebug.el (edebug-wrap-def-body):
* lisp/comint.el (comint-dynamic-list-completions):
* lisp/progmodes/idlwave.el (idlwave-make-modified-completion-map-xemacs)
(idlwave-make-modified-completion-map-emacs)
(idlwave-choose-completion):
* lisp/progmodes/vhdl-mode.el:
* lisp/term.el (term-dynamic-list-completions):
Remove references to 'mouse-choose-completion'.
* lisp/image-mode.el (image-mode-to-text):
Remove reference to 'image-mode-maybe'.
* lisp/mail/rmail.el (rmail-highlight-headers):
Use 'rmail-highlight' face instead of 'rmail-highlight-face'.
* lisp/progmodes/antlr-mode.el (antlr-mode-map, antlr-mode-menu):
Remove reference to 'c-forward-into-nomenclature'.
* lisp/simple.el (choose-completion, choose-completion-string)
(completion-list-mode, completion-setup-function): Don't use
'completion-base-size'.
; * etc/NEWS: List removed items.

This was discussed in
https://lists.gnu.org/archive/html/emacs-devel/2020-08/msg00400.html