Remove compute 2.x rpc api

A previous change adding support for the 3.x rpc interface but retained
compatibility with 2.x as a transition point.  This commit removes the
old API from the server side.

The object_compat decorator is unused as of this commit.  However, I'm
leaving it, because it's going to start getting used very soon again as
more of the compute rpc api gets updated to accept Instance objects.

UpgradeImpact

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

Change-Id: I31c21055163e94b712d337568b16b9b7a224b52f
Merge "Remove compute 2.x rpc api"