#551: fix cylc trigger in simulation mode

This fixes `cylc trigger` in CLI and GUI versions in simulation mode (manual trigger flag was not unset).

It *may* prevent bugs in real time mode, as the `manual_trigger` flag was not unset until after a few time-intensive functions had been called in `submit` (e.g. updating the database, putting messages in a queue). It is now unset straight away. This may have led to the `submit()` function being called multiple times for a single task submit.

See 52207d7.

@arjclark, please review.