Remove support for OSG 3.4
Upstream OSG 3.6 is just as fast, if not faster than OSGoS 3.4; the only changes in OSGoS 3.6 right now are configuration based ones.
The advantages of dropping OSG 3.4 means we drop some pre-processing code and focus just on supporting one version and consigning OSG 3.4 to the past along with 3.2