Remove aggregate_get() from conductor API

The aggregate_get() 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: I893f58bbcae21112f186348cefcbfdd3bcbb8c06
Merge "Remove aggregate_get() from conductor API"