Remove delete_just_stopped_threads_infrun_breakpoints_cleanup

Remove delete_just_stopped_threads_infrun_breakpoints_cleanup

v3: sorted earlier in the series, and replaces the cleanup in
fetch_inferior_event here too instead of in some other patch.

This removes delete_just_stopped_threads_infrun_breakpoints_cleanup,
replacing it with uses of scope_exit.

gdb/ChangeLog:
2019-01-23  Tom Tromey  <tom@tromey.com>
	    Pedro Alves  <palves@redhat.com>

	* infrun.c: Include "common/scope-exit.h"
	(delete_just_stopped_threads_infrun_breakpoints_cleanup): Remove.
	(wait_for_inferior): Use SCOPE_EXIT.
	(fetch_inferior_event): Use scope_exit.