Remove unused methods from backends models

Change-Id: I26f5ecc201883f7f631eeae8c633aa006f923986
Composite PK FederationProtocolModel tests are passing.

Change-Id: I2fae996c41c3a08b8152dc4fa96ea97e7637baf1
Composite PK FederationProtocolModel tests are passing.

Change-Id: I766fd69ceb33acf66323993ca6171eb3e13b70ee
SQL backend primary_keys parameters polished

Change-Id: Icf238833ba18f08acaa4ca6c55ef743f8d21ce7e
SQL backend - ON DELETE CASCADE constraint

Change-Id: I8941d19e2fde529ea6d7da8145059736e1f610ad