vimb-unwrapped: depend on generic webkitgtk instead of 218x

Currently ahead of the upgrade to webkitgtk220x, this will make future
webkitgtk maintenance easier.

WebkitGTK, from 2.6 onwards has maintained API stability and will
continue to do so, as opposed to the jump from 2.4 to 2.6

cc @rickynils

pkgs/top-level/all-packages.nix

···
   };
   vimprobable2 = wrapFirefox vimprobable2-unwrapped { };
 
-  vimb-unwrapped = callPackage ../applications/networking/browsers/vimb {
-    webkit = webkitgtk218x;
-  };
+  vimb-unwrapped = callPackage ../applications/networking/browsers/vimb { };
   vimb = wrapFirefox vimb-unwrapped { };
 
   vips = callPackage ../tools/graphics/vips { };