bug#56502: Lucid ghosts

[Alan Mackenzie]

Hello, Emacs.

While checking for Lisp hooks in the display engine, I came across some
unused variables with "lucid" in their names, namely
Vlucid_menu_bar_dirty_flag and Qrecompute_lucid_menubar.  These could
surely now be removed from Emacs, saving a small amount of space and
execution time.

A grep through the project's sources suggests there are other ghosts with
"lucid", and these could probably also be removed.

-- 
Alan Mackenzie (Nuremberg, Germany).