You can remove an identifier with search/replace. The process is:
- Back up your library in case something goes very wrong
- Select the books with the identifier you wish to remove. The tag browser is one way to do this. Alternatively select all books in the library if you are globally removing an identifier.
- Press E to bulk edit metadata
- Choose the search/replace tab
- Choose regular expression mode
- Choose "identifiers" as the search field. Two "identifier type" boxes will appear.
- Choose the identifier type you wish to remove in the upper identifier type box. Enter that same identifier type into the lower identifier type box.
- Enter .* into the Search for box.
- Leave the Replace with box empty.
- Press OK.
The identifier will be removed from all the selected books.