Remove test_external_network_visibility
Remove test_external_network_visibility

This test was left behind according to plan laid out in patch [1].
Make its removal conditional to the Tempest sync-up patch.

[1] https://review.openstack.org/#/c/280427/

Related-bug: 1553595
Related-bug: 1552960

Change-Id: Ia85e7d03c32b5e2db901aed2c13bafe83896be0b
Depends-on: I0219e17d2ec70939039344826d35cb533b3ce065

bigswitch/neutron,sebrandon1/neutron,MaximNevrov/neutron,mahak/neutron,huntxu/neutron,openstack/neutron,openstack/neutron,mahak/neutron,igor-toga/local-snat,eayunstack/neutron,wolverineav/neutron,openstack/neutron,cloudbase/neutron,cloudbase/neutron,MaximNevrov/neutron,eayunstack/neutron,mahak/neutron,klmitch/neutron,wolverineav/neutron,klmitch/neutron,igor-toga/local-snat,bigswitch/neutron,huntxu/neutron,noironetworks/neutron,noironetworks/neutron,sebrandon1/neutron