docs: drop outdated gallium-docs comment

The separate readthedocs documentation is quite pointless these days, as
it's been moved to docs.mesa3d.org, where all other documentation
already is. There's nothing special about this documentation any longer.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6893>
1 file changed