Remove immediate_quit.

Remove immediate_quit.

The old code that sets and clears immediate_quit was
ineffective except when Emacs is running in terminal mode, and
has problematic race conditions anyway, so remove it.  This
will introduce some hangs when Emacs runs in terminal mode,
and these hangs should be fixed in followup patches.
* src/keyboard.c (immediate_quit): Remove.  All uses removed.