gdb: remove CPP workaround

pkgs/development/tools/misc/gdb/default.nix

···
     ++ stdenv.lib.optional (!pythonSupport) "--without-python"
     ++ stdenv.lib.optional multitarget "--enable-targets=all";
 
-  preConfigure =
-    # Not sure why this is causing problems, now that the stdenv
-    # exports CPP=cpp the build fails with strange errors on darwin.
-    stdenv.lib.optionalString stdenv.cc.isClang ''
-      unset CPP
-    '';
-
   postInstall =
     '' # Remove Info files already provided by Binutils and other packages.
        rm -v $out/share/info/bfd.info