remove README.exabgp
remove README.exabgp

This licence file is no longer relevant, as we are now simply importing
exabgp rather than redistributing it.

stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2,openstack/networking-bagpipe-l2