fix(w.w.template) free queued_updates array

This is a small cleanup to not keep in memory a value we don't need.

After the update_callback callback is called, we don't need to remember
it happens this callback was  queued before. (It also prevents a
possible memory leak (of a boolean value) when widgets are destroyed)

lib/wibox/widget/template.lua

@@ -51,7 +51,7 @@ function template:_do_update_now()
     end
 
     self._private.update_args = nil
-    template.queued_updates[self] = false
+    template.queued_updates[self] = nil
 end
 
 --- Update the widget.