treewide: remove redundant rec

pkgs/development/compilers/llvm/6/compiler-rt.nix

@@ -1,6 +1,6 @@
 { stdenv, version, fetch, cmake, python, llvm, libcxxabi }:
 with stdenv.lib;
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   pname = "compiler-rt";
   inherit version;
   src = fetch "compiler-rt" "1fcr3jn24yr8lh36nc0c4ikli4744i2q9m1ik67p1jymwwaixkgl";

pkgs/development/compilers/llvm/6/libc++/default.nix

@@ -1,6 +1,6 @@
 { lib, stdenv, fetch, cmake, python, libcxxabi, fixDarwinDylibNames, version }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   pname = "libc++";
   inherit version;
 

pkgs/development/compilers/llvm/6/llvm.nix

@@ -25,7 +25,7 @@ let
     imap (i: _: concatStringsSep "." (take i parts)) parts;
 in
 
-stdenv.mkDerivation (rec {
+stdenv.mkDerivation ({
   name = "llvm-${version}";
 
   src = fetch "llvm" "1qpls3vk85lydi5b4axl0809fv932qgsqgdgrk098567z4jc7mmn";