Remove unread-command-char.

Remove unread-command-char.
* src/keyboard.c (read_char, requeued_events_pending_p, Finput_pending_p)
(Fdiscard_input, quit_throw_to_read_char, init_keyboard)
(syms_of_keyboard): Remove support for unread-command-char.
* lisp/emacs-lisp/debug.el (debugger-outer-unread-command-char, debug)
(debugger-env-macro): Remove support for unread-command-char.

* lisp/ehelp.el (with-electric-help): Accept functions in
electric-help-form-to-execute.
(electric-help-execute-extended, electric-help-ctrl-x-prefix): Use it.
And replace unread-command-char -> unread-command-events.

* lisp/subr.el (set-temporary-overlay-map): Minimize slightly the impact of
the temporary map re-appearing on emulation-mode-map-alists.

* lisp/emacs-lisp/edebug.el (def-edebug-form-spec): Remove, it's been broken
since 22.1.