Remove openmmforcefields 13 gaff test code branches

[IAlibay]

trafficstars

Follow up from #893

Once OpenMMForceFields allows gaff for older versions of OpenMM, or once we decide to drop pre 8.1.2 OpenMM versions (whichever comes sooner).

[atravitz]

remove any instances where we're running openmmforcefields 13, since we're now on 14.