Revert "awful.client.movetoscreen: Don't untag clients completely (FS#1196)"

This reverts commit bbe86e4e25. That commit caused
unintuitive, special behavior. Instead, when someone wants us to set a screen,
we really should do so.

lib/awful/client.lua.in

@@ -502,10 +502,8 @@ function client.movetoscreen(c, s)
             s = sel.screen + 1
         end
         if s > sc then s = 1 elseif s < 1 then s = sc end
-        if #tag.selectedlist(s) > 0 then
-            sel.screen = s
-            screen.focus(s)
-        end
+        sel.screen = s
+        screen.focus(s)
     end
 end