units: disable job timeouts

For boot, we might kill fsck in the middle, with likely catastrophic
consequences.

On shutdown there might be other jobs, like downloading of updates for
installation, and other custom jobs. It seems better to schedule an
individual timeout on each one separately, when it is known what
timeout is useful.

Disable the timeouts for now, until we have a clearer picture of how
we can deal with long-running jobs.