Remove HashType::Unknown

Instead, `Hash` uses `std::optional<HashType>`. In the future, we may
also make `Hash` itself require a known hash type, encoraging people to
use `std::optional<Hash>` instead.

src/nix-prefetch-url/nix-prefetch-url.cc

@@ -72,8 +72,6 @@ static int _main(int argc, char * * argv)
             else if (*arg == "--type") {
                 string s = getArg(*arg, arg, end);
                 ht = parseHashType(s);
-                if (ht == HashType::Unknown)
-                    throw UsageError(format("unknown hash type '%1%'") % s);
             }
             else if (*arg == "--print-path")
                 printPath = true;