Remove migration_update() from conductor API

The migration_update() 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: Id385255cbfff8c7a42a9a1551044746b7f6344bd
Merge "Remove migration_update() from conductor API"