BROKER_POOL_LIMIT is causing problem so set it to none

BROKER_POOL_LIMIT is causing random strange connection errors and issues if it's enabled also on the workers and on a single machine (where celeryconfig.py is used by both celery and the openquake main process).

We need to address the bug in another way. See https://bugs.launchpad.net/oq-engine/+bug/1250402