Remove gdb workaround from readline/emacs_keymap.c

Remove gdb workaround from readline/emacs_keymap.c

There is a gdb-local patch in readline/emacs_keymap.c that says:

  /* Temporary - this is a bug in readline 5.1 that should be fixed in
     readline 5.2.  */

So, I think this can be removed now.  I have no way to test this, as
the patch was specific to mingw.

readline/ChangeLog.gdb
2018-10-07  Tom Tromey  <tom@tromey.com>

	* emacs_keymap.c: Remove gdb workaround.