Revert "gtkmm: fix build error by patching missing operator"

This reverts commit 7defda8356.
It breaks the package after update 61c467937b;

(cherry picked from commit 72e0135752)

pkgs/development/libraries/gtkmm/2.x.nix

@@ -9,8 +9,6 @@ stdenv.mkDerivation rec {
     sha256 = "0680a53b7bf90b4e4bf444d1d89e6df41c777e0bacc96e9c09fc4dd2f5fe6b72";
   };
 
-  patches = [ ./gtkmm-2.24.4-missing-operator.patch ];
-
   nativeBuildInputs = [pkgconfig];
 
   propagatedBuildInputs = [ glibmm gtk2 atkmm cairomm pangomm ];

pkgs/development/libraries/gtkmm/gtkmm-2.24.4-missing-operator.patch

@@ -1,11 +0,0 @@
---- gtkmm-2.24.4/gtk/gtkmm/menushell.cc.orig	2016-08-26 21:55:08.858311055 +0000
-+++ gtkmm-2.24.4/gtk/gtkmm/menushell.cc	2016-08-26 21:55:33.206310578 +0000
-@@ -130,7 +130,7 @@ MenuList::iterator MenuList::insert(Menu
- {
-   const Glib::RefPtr<Gtk::MenuItem> item = element.get_child();
- 
--  g_return_val_if_fail(item != 0, position);
-+  g_return_val_if_fail(!!item, position);
-   g_return_val_if_fail(gparent() != 0, position);
- 
-   int pos = -1;