treecompose-post.sh: no longer manually prune locales
Now that rpm-ostree correctly handles the install-langs key, the glibc
%post script automatically does the locale pruning for us. This is
similar to what we did in F25, but we don't prune non-UTF8 locales here
just because it's a bit late to change that for F24.
This was causing F24 composes to fail because the grep -a -v wouldn't
find any matches (because glibc already pruned everything for us).