Remove redundant initialization and check from DVR RPC mixin

The initialization, as well as the recurrent checks on whether the agent
is able to handle DVR requests, are not necessary; this is because the
dvr_agent is going to be set at the time the L2 agent starts receiving
RPC messages.

Change-Id: I9caad965d0ff507e2ed93d6432997ad86139ac04
Merge "Remove redundant initialization and check from DVR RPC mixin"