x11: Remove DRI2 support

DRI2 was hidden behind a legacy-x11 build option back in the 24.2
release. It's now been a year, so let's get on with removing it so we
can simplify our winsys interactions.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35885>

.gitlab-ci/build/gitlab-ci.yml

@@ -188,7 +188,6 @@ debian-build-x86_64:
       -D egl=enabled
       -D glvnd=disabled
       -D platforms=x11,wayland
-      -D legacy-x11=dri2
     GALLIUM_ST: >
       -D gallium-extra-hud=true
       -D gallium-vdpau=enabled