core: remove system start timeout logic again

core: remove system start timeout logic again

The system start timeout as previously implemented would get confused by
long-running services that are included in the initial system startup
transaction for example by being cron-job-like long-running services
triggered immediately at boot. Such long-running jobs would be subject
to the default 15min timeout, esily triggering it.

Hence, remove this again. In a subsequent commit, introduce per-target
job timeouts instead, that allow us to control these timeouts more
finegrained.