pin-to-gcc12-if-gcc13: drop as it's not used anymore

pkgs/top-level/all-packages.nix

@@ -15675,14 +15675,6 @@ with pkgs;
   gccFun = callPackage ../development/compilers/gcc;
   gcc-unwrapped = gcc.cc;
 
-  pin-to-gcc12-if-gcc13 = pkg:
-    if !(lib.isDerivation pkg) || !(pkg?override) then pkg else
-    pkg.override (previousArgs:
-      lib.optionalAttrs (previousArgs.stdenv.cc.cc.isGNU or false &&
-                         lib.versionAtLeast previousArgs.stdenv.cc.cc.version "13.0") {
-                           stdenv = gcc12Stdenv;
-                         });
-
   wrapNonDeterministicGcc = stdenv: ccWrapper:
     if ccWrapper.isGNU then ccWrapper.overrideAttrs(old: {
       env = old.env // {