Removing a router twice from the same agent shouldn't cause an error

Removing a router twice from the same agent shouldn't cause an error

When we remove a router from an agent that has already been
unscheduled from we raise an exception that eventually causes an error.
The method '_unbind_router' raises a 'RouterNotHostedByL3Agent' exception
on failure. In both cases the actual removal of the router
from the same agent has no effect.

The solution is to stop raising 'RouterNotHostedByL3Agent' so
that _unbind_router() being invoked without error can indicate that
the router is no longer bound.

This solution matches the behaviour found when trying
to schedule a router to the same agent twice.

This change is a result of the discussion in:

https://review.openstack.org/#/c/144681/2

Closes-Bug: #1406705

Change-Id: I015bfc0fde11ba4f39417e4c134faa8132cb3eac