src/*.h: Remove unused parameters and functions.

src/*.h: Remove unused parameters and functions.

* keyboard.h (timer_check, show_help_echo): Remove unused parameters.

* keyboard.c (timer_check): Remove parameter `do_it_now',
  unused since revno:14998 (1996-04-12).
  (show_help_echo): Remove parameter `ok_to_overwrite_keystroke_echo',
  unused since revno:86854 (2008-04-19).

* keyboard.c (read_char):
* w32menu.c (w32_menu_display_help):
* xmenu.c (show_help_event, menu_help_callback):
  Adjust calls to `show_help_echo'.

* gtkutil.c (xg_maybe_add_timer):
* keyboard.c (readable_events):
* process.c (wait_reading_process_output):
* xmenu.c (x_menu_wait_for_event): Adjust calls to `timer_check'.

* insdel.c (adjust_markers_gap_motion):
  Remove; no-op since revno:20569 (1998-01-02).
  (gap_left, gap_right): Don't call it.