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

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: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Emil Velikov <emil.velikov@collabora.com>