fix(mute-hashtag): clear the filter optional when successfully deleti…

…ng the hashtag filter on the server

mastodon/src/main/java/org/joinmastodon/android/fragments/HashtagTimelineFragment.java

@@ -170,6 +170,7 @@ private void unmuteHashtag() {
 		new DeleteFilter(filter.get().id).setCallback(new Callback<>(){
 			@Override
 			public void onSuccess(Void result){
+				filter=Optional.empty();
 				updateMuteState(false);
 			}