[Trunk] Delete selected measures in first bar and undo introduces courtesy clef(s)

1. Create piano score.
2. Enter notes in first bar.
3. Drag-select a note.
4. Delete selected measures.
5. Undo.

Result: The second bar has courtesy clef(s).

Using MuseScore 2.0 Nightly Build (4367) - Mac 10.6.7.