rtp_engine and stun: call ast_register_atexit instead of ast_register_cleanup

rtp_engine and stun: call ast_register_atexit instead of ast_register_cleanup

rtp_engine.c and stun.c were calling ast_register_cleanup which
is skipped if any loadable module can't be cleanly unloaded
when asterisk shuts down.  Since this will always be the case,
their cleanup functions never get run.  In a practical sense
this makes no difference since asterisk is shutting down but if
you're in development mode and trying to use the leak sanitizer,
the leaks from both of those modules clutter up the output.