Remove OSVR from jme3-VR

[richardTingle]

As discussed at https://hub.jmonkeyengine.org/t/should-osvr-very-old-vr-standard-be-removed-from-jme/48234 OSVR is long deprecated and unlikely to have any users. It should be removed

[stephengold]

With the release of JME v3.8.0-alpha4, I believe this issue can be closed.