fix: make polymorphic relationhip name consistent

[mindok]

Fixes #190

Removes incorrect output from AshPostgres.DataLayer.Info.polymorphic_name(relationship.source) from possible name of reference / constraint.

[zachdaniel]

So, the fix for this was to properly honor that configuration instead of having it pull in the on_delete option. Thanks for the fix, pushed something else to address the issue.