Move block_device_mapping destroy operations to conductor

This patch moves the compute/manager's use of block_device_mapping_destroy
operations to the conductor. Specifically:

 - block_device_mapping_destroy()
 - block_device_mapping_destroy_by_instance_and_device()
 - block_device_mapping_destroy_by_instance_and_volume()

Once again, this just adds a single destroy operation to the conductor's
RPC API and replicates the older interfaces in the public API. Further,
the first now takes a list of bdms to be destroyed (unlike the db
operation it mirrors) so that multiple RPC calls can be avoided for
bulk delete operations.

Related to blueprint no-db-compute-manager

Change-Id: Ib93184ee6d32fcda90f75a26b579787e4625e9ae
Merge "Move block_device_mapping destroy operations to conductor"