Eliminate clear_quit_flag

Eliminate clear_quit_flag

Nothing calls this anymore.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* defs.h (clear_quit_flag): Remove declaration.
	* extension-priv.h (struct extension_language_ops)
	<clear_quit_flag>: Remove field and update comments.
	* extension.c (clear_quit_flag): Delete.
	* guile/guile.c (guile_extension_ops): Adjust.
	* python/python.c (python_extension_ops): Adjust.
	(gdbpy_clear_quit_flag): Delete.