Database Models: Remove obsolete signals

Folder model and comic model contain two signal relais which do not
serve any obvious purpose.
Remove them.

YACReaderLibrary/db/comic_model.h

@@ -160,8 +160,6 @@ private:
     qulonglong sourceId;
 
 signals:
-    void beforeReset();
-    void reset();
     void isEmpty();
     void searchNumResults(int);
     void resortedIndexes(QList<int>);