tevent: simplify tevent_cleanup_pending_signal_handlers()

Calling tevent_signal_destructor() does the same as se->event_ctx is already
NULL.

This also makes sure we correctly cleanup the SA_SIGINFO array.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
