texlive: Use lib.unique for uniqueness checks

In 85605c8a296bd54e9c12e1bd1c33bf1e100d40b7 we got a really nice
performance improvement.

By using this implementation for texlive instantiation RSS is cut by
~2.6x with a similar wall time speedup.

pkgs/tools/typesetting/tex/texlive/combine.nix

@@ -31,13 +31,7 @@ let
       ++ lib.optional (lib.any pkgNeedsRuby splitBin.wrong) ruby;
   };
 
-  # TODO: replace by buitin once it exists
-  fastUnique = comparator: list: with lib;
-    let un_adj = l: if length l < 2 then l
-      else optional (head l != elemAt l 1) (head l) ++ un_adj (tail l);
-    in un_adj (lib.sort comparator list);
-
-  uniqueStrings = fastUnique (a: b: a < b);
+  uniqueStrings = list: lib.sort (a: b: a < b) (lib.unique list);
 
   mkUniqueOutPaths = pkgs: uniqueStrings
     (map (p: p.outPath) (builtins.filter lib.isDerivation pkgs));