docs: remove documentation of MESA_HPCR_CLEAR

We haven't respected this variable for about 10 years, as the support
was removed in bc63b226bcc ("xlib: remove a ton of old xlib driver
cruft").

So let's also remove the documentation.

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