stdenv/adapters.nix: remove static from makeStaticLibraries

This logic should be in the pkgs/top-level/static.nix. We don’t want
to pollute Nixpkgs with =if stdenv.static=. Also, "static" is not
descriptive. We have two types of static stdenvs, ‘makeStaticLibaries’
and ‘makeStaticBinaries’. We shouldn’t rely on a static boolean like
this.