This option was deprecated since couple of releases already.
In Stein we removed 'external_network_bridge' option from L3 agent's
config so now it's time to remove also this one.
There is also new upgrade check introduced to check and warn
users if gateway_external_network_id was used in the deployment.
This patch also removes method _check_router_needs_rescheduling() from
neutron/db/l3_db.py module as it is not needed anymore.
This patch also removes unit tests:
test_update_gateway_agent_exists_supporting_network
test_update_gateway_agent_exists_supporting_multiple_network
test_router_update_gateway_no_eligible_l3_agent
from neutron/tests/unit/extensions/test_l3.py module as those
tests are not needed when there is no "gateway_external_network_id"
config option anymore.
Change-Id: Id01571cd42cfe9c5ce91e90159917c7d3c963878