v3dv/pipeline: remove pregenerate_variant

v3dv/pipeline: remove pregenerate_variant

Right now we were not pre-generating several variants, but we decided
to let this method, just in case we need that idea back. This ended
being a bad idea. Several months have passed without that need, so
having that method just adds confusion. Also, if we need to add a
multiple-variant in the future, perhaps we would need to do it
different, so let's not template in advance.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9403>