mapi: Drop dynamic dispatch stub generation.

Since we enforce that the frontend (src/{glx,egl}) matches the driver (src/mesa/), we also know that src/mapi's libglapi matches between the two sides. This lets us drop a bunch of untested code for dynamic dispatch stub generation to handle new-mesa vs old-libgl that we don't need any more.

I think we could take a step farther and have _gloffset_* statically be the offsets in libglapi instead of having to query them. But -1100 lines is pretty good for one day.