Remove legacy report_state callback

Deleted AgentExtRpcCallback
Unregister AgentExtRpcCallback in PluginRpcDispatcher in all core plugins
Changed target topic of PluginReportStateAPI in all remaining agents

TODO: Remove AgentExtRpcCallback related unittests

Change-Id: Iecd3a2cbd8af2a2fc6a09b009fbf8226881e3815