LP1993922: Course material delete should not delete other courses' materials

LP1993922: Course material delete should not delete other courses' materials

To test:
* Apply this patch
* Login to BR1 workstation
* Go to Admin - Local Admin - Course Reserves List
* Create two courses, Course 1 and Course 2
* Associate materials to both courses
* Archive Course 1
* Go to Course materials on Course 2 and note that
it still has all of its materials attached.

Signed-off-by: Jane Sandberg <js7389@princeton.edu>
Signed-off-by: Beth Willis <willis@noblenet.org>
Signed-off-by: Michele Morgan <mmorgan@noblenet.org>