Don't fail when trying to unbind a router

If a router is already unbound from an l3 agent, don't fail.  Log
the condition and go on.  This is harmless since it can happen
due to a delete race condition between multiple neutron-server
processes.  One delete request can determine that it needs to
unbind the router.  A second process may also determine that it
needs to unbind the router.  The exception thrown will result
in a port delete failure and cause nova to mark a deleted instance
as ERROR.

Change-Id: Ia667ea77a0a483deff8acfdcf90ca84cd3adf44f
Closes-Bug: 1367892