Revert "FIX: pinned-button sometimes not shown when unpinned is null"

This reverts commit 01f0b56331.

app/assets/javascripts/discourse/components/pinned-button.js.es6

@@ -17,6 +17,6 @@ export default Ember.Component.extend({
 
   @computed("topic.pinned", "topic.deleted", "topic.unpinned")
   hidden(pinned, deleted, unpinned) {
-    return deleted || (pinned === false && unpinned === false);
+    return deleted || (!pinned && !unpinned);
   }
 });