Remove prepare_re_set_context

prepare_re_set_context returns a null cleanup and doesn't seem
generally useful.  This patch removes it plus a few more cleanups; and
changes breakpoint_re_set to use scoped_restore rather than its own
manual mechanism.

2017-10-11  Tom Tromey  <tom@tromey.com>

	* breakpoint.c (prepare_re_set_context): Remove.
	(breakpoint_re_set_one): Update.  Don't use cleanups.
	(breakpoint_re_set): Use scoped_restore, std::string, and
	scoped_restore_current_language.