manager: remove transient unit directory during startup

manager: remove transient unit directory during startup

I was testing transient units and user@.service crashed. I restarted it, and
tried to create a transient unit. It failed because
/run/user/1000/systemd/transient/ remained after the previous aborted run:
  Failed to start transient service unit: Unit run-u0.service was already loaded or has a fragment file.

Remove the directory during initial startup so we don't get confused by our own
files.