timesyncd,resolved,machinectl: drop calls to sd_event_get_exit_code()

timesyncd,resolved,machinectl: drop calls to sd_event_get_exit_code()

In all three cases, sd_event_loop() will return the exit code anyway.
If sd_event_loop() returns negative, failure is logged and results in an
immediate return. Otherwise, we don't care if sd_event_loop() returns 0
or positive, because the return value feeds into DEFINE_MAIN_FUNCTION(), which
doesn't make the distinction.