Removed __glDispatchGetOffset and __glDispatchSetEntry from libGLdispatch.
Neither one is used anywhere, and the dispatch offsets should be an internal
detail of libGLdispatch.

Removed the mutex from stub_find_dynamic. Anything that calls that function
should already be holding the GLdispatch lock.

Also removed the __GLdispatchGetOffsetHook, since nothing uses it anymore.