Resolve the broken coding pipeline in ext-inter experiment when
cb4x4 mode is enabled. Turn off rectangular inter-intra mode.
This needs some more work to hook up. Given that it gives fairly
limited coding performance gains, disable it for the moment.

BUG=aomedia:309

Change-Id: I9b406df6183f75697bfd4eed5125a6e9436d84b0