glx: drop misleading comment about the file being "generated"

This gen_scrn_dispatch.pl has never existed, in the sense that NVIDIA never published it. There have been a number (6) of commits to fix various things in there over the years, and never anything from NVIDIA.

For all intents and purposes this file is hand-written and hand-maintained, and we're on our own.

Let's make this clear by removing this misleading comment.

Suggested-by: @anholt
/cc @ajax @evelikov @jwrdegoede @sthibaul, who have touched this file over the years since NVIDIA dumped it in our codebase