The externally consumed APIs from neutron.db.api were rehomed into
neutron-lib with https://review.openstack.org/#/c/557040/
This patch consumes the retry_db_errors function from lib by:
- Removing retry_db_errors from neutron.db.api
- Updating the imports for retry_db_errors to use it from lib
- Using the DB API retry fixture from lib in the UTs where applicable
- Removing the UTs for neutron.db.api as they are now covered in lib
NeutronLibImpact
Change-Id: I1feb842d3e0e92c945efb01ece29856335a398fe