Remove folsom-specific logic

No testing is done on OpenStack Folsom any longer, so we can clean
up devstack-gate scripts accordingly.

* devstack-vm-gate-dev.sh
* devstack-vm-gate-wrap.sh
* devstack-vm-gate.sh: Remove conditionals and variables relevant
only to stable/folsom testing.

Change-Id: I4e09c241920fb1c35138d6f34fc889b10926a0e6
Merge "Remove folsom-specific logic"