The public db apis are available in neutron-lib. This patch consumes
the sqla_listen, sqla_remove and sqla_remove_all functions from lib
by removing them from neutron and using lib's version.
There's also a TODO added here so we can eventually rehome the
check_no_sqlalchemy_event_import hacking check to neutron-lib.
NeutronLibImpact
Change-Id: I3b3862016125303fb227b4ff41420a89bb33c917