Revert unlicensed contribution

Some explanation is in order:

I have tried to contact Zanculmarktum three times in the last
25 days, but I have not received any response.
Meanwhile, others are waiting to be able to use flake-compat, so
I have decided that it is best to move forward with the licensing
effort, considering that everyone else except Eelco has already
agreed, and quickly as well.

I have no hard feelings towards Zanculmarktum and I really do not
like having to make this commit.

This reverts commit e363cffac2 and
subsequent improvements.

I should note that it could be argued that their contribution was
not a creative work in the sense of most copyright law, but I'd
prefer to avoid any confusion by choosing to reimplement their
contribution in a "clean room" manner.

default.nix

@@ -46,13 +46,6 @@ let
       { outPath = builtins.path { path = info.path; };
         narHash = info.narHash;
       }
-    else if info.type == "tarball" then
-      { outPath =
-          fetchTarball
-            ({ inherit (info) url; }
-             // (if info ? narHash then { sha256 = info.narHash; } else {})
-            );
-      }
     else if info.type == "gitlab" then
       { inherit (info) rev narHash lastModified;
         outPath =