element.py: Remove _get_artifact_cache() accessor.

The artifact cache is anyway a singleton, the Element itself
has an internal handle on the artifact cache because it is
needed very frequently; but an artifact cache is not element
specific and should not be looked up by surrounding code
on a per element basis.

Updated _scheduler/queues/queue.py, _scheduler/queues/buildqueue.py
and _scheduler/jobs/elementjob.py to get the artifact cache directly
from the Platform