Remove firefox, xvfb from bindep-fallback
Remove firefox, xvfb from bindep-fallback

We now have repositories with other-requirements that include these
packages, so the files will be cached on our daily builds.

Currently, we install these packages in jobs where they are needed, so
there is no need to install these on each and every non-devstack build.

Change-Id: Ia51738950b4d3e2e2b82107d437fc167cb9e1282

Tesora/tesora-project-config,Tesora/tesora-project-config,dongwenjuan/project-config,openstack-infra/project-config,dongwenjuan/project-config,openstack-infra/project-config