Remove `Column#primary`

It appears to have been used at some point in the past, but is no longer
used in any meaningful way. Whether a column is considered primary is
a property of the model, not the schema/column. This also removes the
need for yet another layer of caching of the model's schema, and we can
leave that to the schema cache.
Merge pull request #15282 from sgrif/sg-remove-column-primary

Remove `Column#primary`