bus-util: Drop fallback to system/user bus if manager bus doesn't work

bus-util: Drop fallback to system/user bus if manager bus doesn't work

We have various callsites that explicitly need the manager bus and
won't work with the system bus, like daemon-reexec and friends which
can't properly wait until the operation has finished unless using the
manager bus.

If we silently fall back to the system bus for these operations, we
can end up with rather hard to debug issues so let's remove the fallback
as it was added back in 2013 in a6aa89122d2fa5e811a72200773068c13bfffea2
without a clear explanation of why it was needed (I expect as a fallback
if kdbus wasn't available but that's not a thing anymore these days).