Remove implicit registration of *-aas service providers

Remove implicit registration of *-aas service providers

Implicit registration can be dropped when explicit registration
for load balancer and vpn is implemented. Firewall does not
use service providers and the ServiceTypeManager, so the
precautionary step can be dropped altogether.

Support for configuring providers via the service_providers section
in neutron.conf, is no longer available, hence clear the stale
entry points.

DocImpact

Closes-bug: #1473110

Change-Id: I5e1d254b9a3a24121d9e9d3cb82f877d44079296