Bug 16015: Restore the ability to delete a group of libraries

Bug 16015: Restore the ability to delete a group of libraries

If a group of libraries is linked to at least 1 library, the group
cannot be delete and the librarian should get a warning message.
But if no libraries are linked, the deletion should be allowed.
Since bug 15294 this behavior is broken: The deletion is always blocked:
"This library category cannot be deleted. 0 libraries are still using
it", hum...

Test plan:
Create a group of libraries, delete it

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com