Use defvar-keymap for some trivial keymaps

Use defvar-keymap for some trivial keymaps

* lisp/bindings.el (mode-line-window-dedicated-keymap)
(mode-line-buffer-identification-keymap):
* lisp/emulation/cua-rect.el (cua--overlay-keymap, cua--overlay-key):
* lisp/mh-e/mh-mime.el (mh-mime-security-button-map):
* lisp/mh-e/mh-utils.el (mh-hidden-header-keymap):
* lisp/net/eudc-bob.el (eudc-bob-generic-keymap, eudc-bob-image-keymap)
(eudc-bob-sound-keymap, eudc-bob-url-keymap, eudc-bob-mail-keymap):
* lisp/progmodes/etags-regen.el (etags-regen-mode-map):
* lisp/progmodes/octave.el (octave-help-mode-map):
* lisp/replace.el (multi-query-replace-map):
* lisp/simple.el (process-menu-mode-map, messages-buffer-mode-map):
* lisp/startup.el (splash-screen-keymap):
* lisp/tab-bar.el (tab-bar-mode-map):
* lisp/textmodes/ispell.el (ispell-minor-keymap):
* lisp/textmodes/tex-mode.el (latex-mode-map, plain-tex-mode-map):
* lisp/tree-widget.el (tree-widget-button-keymap):
* lisp/vc/vc-hooks.el (vc-mode-line-map): Use defvar-keymap.

(cherry picked from commit d8d524071d78d6499278546cf13ae261c05cadf3)