Expunge unused recompute-lucid-menubar, lucid-menu-bar-dirty-flag

Expunge unused recompute-lucid-menubar, lucid-menu-bar-dirty-flag

This fixes bug #56502.  That function and these variables were manipulated by
the former file emacs-lisp/lmenu.el that was finally expunged from directory
obsolete/ by Stefan Kangas on 2020-05-15.  There remain references to them in
the *.c, and *.m files.

* src/keyboard.c (syms_of_keyboard): Remove declarations of the symbol and
variable.

* src/haikumenu.c (set_frame_menu_bar)
* src/keyboard.c (command_loop_1)
* src/nsmenu.m (ns_update_menubar)
* src/pgtkmenu.c (set_frame_menubar)
* src/xdisp.c (update_menu_bar)
* src/xmenu.c (set_frame_menubar): Remove calls to Qrecompute_lucid_menubar
contitional on Vlucid_menu_bar_dirty_flag.