neutron-legacy: Remove no longer necessary vpnaas conditional

VPNaaS agent is going to be an L3 agent extention.

Related-Bug: #1692128
Depends-On: I0b86c432e4b2210e5f2a73a7e3ba16d10467f0f2
Change-Id: Id827274b7c74cdf71db6d1f2ab3eadb5fef099f5
1 file changed