More schema cleanup

This cleans the rest of the cruft & unused columns out of the schema, and switches the BAO to fully use standard CRUD methods.

Added a pseudoconstant and FK declaration for the entity_id+entity_table columns which allows SearchKit to perform joins on them.

TODO: I tried writing an upgrader to drop the unused columns if they exist, and failed. It kept crashing with an infinite loop. I'd like to try again but want to wait for the work in civix to mature for using the core SchemaTrait which provides helper functions for adding/dropping columns. In the meantime, this can be merged as the columns aren't hurting anything.