The previous commits have removed all usage of CaQL. It's no longer
needed.

The feature was disabled, by removing it from guc.c, only months after
it was introduced, back in 2009. I'm not sure if it ever made it into a
public release. We have no plans to resurrect it, and it has surely
bitrotted so badly that even if we did, it would be better to start from
scratch.

Codes of ic_tcp and ic_udp are not maintained any more, ic_udpifc is
the only interconnect type for now. We keep the gp_interconnect_type
guc for backward compatibility.