Make Field.db_parameters() use db_type()

This reverts parts of ca9c3cd, which appear to have been made unintentionally.

It removes support for `db_type` to return the "CHECK (%s)" constraint suffix.
Internal code appears to use `db_parameters` for this.

Fixes https://code.djangoproject.com/ticket/22260.

This needs tests. Please point me at where to put them, and how to test this best.