docs: remove documentation of MESA_CI_VISUAL

The support for color-indexed rendering was removed from mesa all the
way back in 2010, in commit 81fe9949ef0 ("mesa/xlib: Remove support for
color-index rendering").

So let's drop it from our documentation as well. This environment
variable does nothing.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10281>
2 files changed