wxwidgets: remove unused asserts

pkgs/development/libraries/wxwidgets/wxGTK28.nix

···
 , withMesa ? lib.elem stdenv.hostPlatform.system lib.platforms.mesaPlatforms
 }:
 
-assert withMesa -> libGLU != null && libGL != null;
-
 stdenv.mkDerivation rec {
   pname = "wxGTK";
   version = "2.8.12.1";

pkgs/development/libraries/wxwidgets/wxGTK29.nix

···
 , darwin
 }:
 
-assert withMesa -> libGLU != null && libGL != null;
 let
   inherit (darwin.stubs) setfile;
   inherit (darwin.apple_sdk.frameworks) AGL Carbon Cocoa Kernel QuickTime;

pkgs/development/libraries/wxwidgets/wxGTK31.nix

···
 , darwin
 }:
 
-assert withMesa -> libGLU != null && libGL != null;
-assert withWebKit -> webkitgtk != null;
-
 assert withGtk2 -> (!withWebKit);
 
 let