libsigcxx: removed patch that forced -std=c++11 in pkg-config file

This commit removes a patch from the derivation for libsigc++-2.0 that
appended -std=c++11 to the pkg-config file.

The patch in question orginated from the Debian project to enforce
-std=c++11 in projects using libsigc++-2.0 via pkg-config. This patch
was removed in 2016 when Debian switched to GCC 6.

This fixes NixOS/nixpkgs#33534.

pkgs/development/libraries/libsigcxx/default.nix

@@ -10,11 +10,6 @@ stdenv.mkDerivation rec {
     url = "mirror://gnome/sources/libsigc++/${ver_maj}/${name}.tar.xz";
     sha256 = "f843d6346260bfcb4426259e314512b99e296e8ca241d771d21ac64f28298d81";
   };
-  patches = [(fetchpatch {
-    url = "https://anonscm.debian.org/cgit/collab-maint/libsigc++-2.0.git/plain"
-      + "/debian/patches/0002-Enforce-c-11-via-pkg-config.patch?id=d451a4d195b1";
-    sha256 = "19g19473syp2z3kg8vdrli89lm9kcvaqajkqfmdig1vfpkbq0nci";
-  })];
 
   nativeBuildInputs = [ pkgconfig gnum4 ];