dri: delete gen-symbol-redefs.py

Introduced in ba10d79c but it looks like it was never wired into anything.

I'm assuming it's just dead code by now?

I'm looking at all the python2 code left in Mesa and that's the last python2-shebang script in the tree other than src/gallium/tools/trace/*.py.

/cc @anholt @mattst88 @evelikov