Get rid of the last remaining uses of var_is_rel(), to wit some debugging

Get rid of the last remaining uses of var_is_rel(), to wit some debugging
checks in ExecIndexBuildScanKeys() that were inadequate anyway: it's better
to verify the correct varno on an expected index key, not just reject OUTER
and INNER.

This makes the entire current contents of nodeFuncs.c dead code.  I'll be
replacing it with some other stuff later, as per recent proposal.