diff-mode-shared-map: Don't inherit from special-mode-map

diff-mode-shared-map: Don't inherit from special-mode-map

* lisp/vc/diff-mode.el (diff-mode-shared-map): No longer inherit
from special-mode-map.
(diff-mode-map): Tidy up list of unbindings now that we no
longer need to override any bindings from special-mode-map.
(minor-mode-map-alist): <diff-mode-read-only>: Ensure that the
minor mode keymap continues to inherit from special-mode-map by
calling make-composed-keymap here.

Co-authored-by: Stefan Monnier <monnier@iro.umontreal.ca>
(cherry picked from commit 4ddafede8d7bebdaa795279cc4c527e5fe425add)