Remove reference tables from pg_dist_colocation table

We added a new UDF to create reference tables with #892. We also use a valid colocation id for reference tables to determine reference tables by quickly checking the shard count on pg_dist_colocation table. Though, the name of pg_dist_colocation becomes a bit misleading because there is no colocation concept for reference tables.

Two options are;
i. Renaming pg_dist_colocation to something like pg_dist_table_configuration or
ii. Finding another way to figure out reference tables

Related to #879

Start by reviewing the reference-table UDF work from #892 and the related discussion in #879. The issue does not name implementation files or tests; determine whether pg_dist_colocation should be renamed or replaced, and consider the work done when reference tables no longer rely on a misleading colocation concept.

Written by the indexing model from the issue text.