Remove service start timeout

On a slow machine the service may take longer than 5 seconds (walltime) to start,
especially during session start when multiple processes are competing for
CPU time and I/O.

The original reasoning for the timeout is to limit the time to quit the process
when the session is shut down, but that can be set with TimeoutStopSec.

Whether the start timeout is useful at all is questionable. It only helps if
there is some deadlock during startup which would be cured by a fresh
start, otherwise it is just consuming CPU time again and again.

For comparision, Gnome does not use Timeout[Start]Sec at all, but
only TimeoutStopSec.

CCBUG: 477805