Bug 10382: collection not returning to null when removed from course reserves

Bug 10382: collection not returning to null when removed from course reserves

Test Plan:
1) Create an item, do not set a collection code
2) Add the item to a course, and choose to set a collection code
3) Ensure the course is enabled, and the collection code is now visible
4) Disable the course, ensure the collection code is no longer visible

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>