Remove block_device_mapping_destroy() from conductor API

The block_device_mapping_destroy() method of the conductor API is no
longer used.  Remove it from the client code and leave a note on the
manager side for it to be removed in the next major version bump.

This is cleanup, but part of a blueprint being completed during
feature freeze.  It allows us to drop more code in the rpc version
bump.

Part of blueprint rpc-major-version-updates-icehouse

Change-Id: Iffdb6f77cd611918ca27f64162c1b4f13a46815b
Merge "Remove block_device_mapping_destroy() from conductor API"