AUP3: Don't delete sample blocks prematurely

The wrong sample blocks would be deleted at close in step 6:

1) New Project
2) Save and name
3) Import one stereo MP3 (about 6 minutes)
4) Save, close and re-open
5) Apply Bass and Treble to the track using real-time preview
6) Save, close and re-open
7) Message Failed to retrieve sample block

src/ProjectManager.cpp

@@ -748,8 +748,6 @@ void ProjectManager::OnCloseWindow(wxCloseEvent & event)
 
       // Delete all the tracks to free up memory
       tracks.Clear();
-
-      trans.Commit();
    }
 
    // We're all done with the project file, so close it now