Here's the approach that may work for you.

1. Menu Navigate -> Entire collection.
2. Menu View -> Table view (unless already activated).
3. Right-click -> Columns. Add Personal Notes columns to the list and press OK. You will see a new column that contains the notes.
4. Sort list by Personal notes column (click on the header). Now you can see albums with similar notes (like MusicMatch_Preference) are grouped together.
5. Using Shift key, select all albums that have MusicMatch notes. Right-click the selected albums and click Properties.
6. Switch to the Notes tab and delete everything from Personal notes field. Careful! You are editing many albums at the same time, so don't do any other changes!
7. Press OK.

Andrei