uv-resolver: remove 'fork.is_false()' filtering
This filtering is now redundant, since forking now avoids these
degenerate cases by construction.
The main change to forking that enables skipping over "always
false" forks is that forking now starts with the parent's markers
instead of starting with MarkerTree::TRUE and trying to combine
them with the parent's markers later. This in turn leads to
skipping over anything that "can't" happen when combined with the
parents markers. So we never hit the case of generating a fork
that, when combined with the parent's markers, results in a
marker that is always false. We just avoid it in the first place.
