Remove unused problematic input nix-tools (#1736)

Author: hamishmack

flake.nix

@@ -46,7 +46,6 @@
       url = "github:sevanspowell/hpc-coveralls";
       flake = false;
     };
-    nix-tools = { url = "path:./nix-tools"; };
     old-ghc-nix = {
       url = "github:angerman/old-ghc-nix/master";
       flake = false;