Remove neutron.common.rpc.RPCException

Remove RPCException, which was just mapped directly to
oslo.messaging.MessagingException for the purposes of minimizing the
impact to the code base when moving from openstack.common.rpc to
oslo.messaging.

Part of blueprint drop-rpc-compat.

Change-Id: Idc5af503f24217fbd2d982c338434c200d025521
Merge "Remove neutron.common.rpc.RPCException"