Revert "gcc: Use the pre-C++11 ABI by default"

This reverts commit 83011723af. This
never worked because gcc's setup hook is never actually sourced. I
could fix this but since nobody complained, we probably don't need it.

pkgs/development/compilers/gcc/5/default.nix

@@ -495,8 +495,6 @@ stdenv.mkDerivation ({
 
   inherit (stdenv) is64bit;
 
-  setupHook = ./use-old-abi.sh;
-
   meta = {
     homepage = http://gcc.gnu.org/;
     license = stdenv.lib.licenses.gpl3Plus;  # runtime support libraries are typically LGPLv3+

pkgs/development/compilers/gcc/5/use-old-abi.sh

@@ -1 +0,0 @@
-NIX_CFLAGS_COMPILE+=" -D_GLIBCXX_USE_CXX11_ABI=0"