dri: get rid of LIBGL_SHOW_FPS

The same functionnality can be achieved using GALLIUM_HUD=stdout,fps (and for
now a fallback is doing this if LIBGL_SHOW_FPS=1 is used).

This removes one entry from the vtable and simplify dri3_handle_present_event.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20494>

docs/envvars.rst

@@ -30,11 +30,6 @@ LibGL environment variables
    if set to ``true``, do not use DrawArrays GLX protocol (for
    debugging)
 
-.. envvar:: LIBGL_SHOW_FPS
-
-   print framerate to stdout based on the number of ``glXSwapBuffers``
-   calls per second.
-
 .. envvar:: LIBGL_DRI2_DISABLE
 
    disable DRI2 if set to ``true``.