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

(cherry-picked from commit cc863e07156e3cfe020c5acfcd9875daff477de6)
Change-Id: Ia85e7d03c32b5e2db901aed2c13bafe83896be0b
Depends-on: I0219e17d2ec70939039344826d35cb533b3ce065

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