stop throwing an exception in DBHelper.onDowngrade()

I missed this little detail ind64a55e013882a7d6b3de646955ed68647a82e97,
the super version of this throws an exception, so it stops the downgrade.

fdroid/fdroidclient!729

app/src/main/java/org/fdroid/fdroid/data/DBHelper.java

@@ -280,7 +280,6 @@ public class DBHelper extends SQLiteOpenHelper {
 
     @Override
     public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
-        super.onDowngrade(db, oldVersion, newVersion);
         resetTransient(context);
     }