Refactor check_functional_grouping() to use get_primary_key_attnos().

Refactor check_functional_grouping() to use get_primary_key_attnos().

If we ever get around to allowing functional dependency to be proven
from other things besides simple primary keys, this code will need to
be rethought, but that was true anyway.  In the meantime, we might as
well not have two very-similar routines for scanning pg_constraint.

David Rowley, reviewed by Julien Rouhaud