graphics/mesa-devel: drop Clover from OPENCL option

Pending https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19385
but after https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27783
is no longer supported on Intel GPUs. Instead of continuing blindly simply
drop the feature. This port is limited to dogfood-ready features, anyway.