Bug 32437: Cleanup script

This patch:
- fixes some indentation
- flips unless-else to if-else for readability
- expands some comments for clarity
- removes unused biblioitemnum variable

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>