LP#937789: fake-delete support for monograph parts

LP#937789: fake-delete support for monograph parts

Unlike copies, call numbers, and bibs, parts can be directly removed from the
database.  This has a negative impact on holds, in that the holds become not
just orphaned, but broken entirely.  With this commit we bring part holds to
parity with other hold types such that they can be system-canceled when their
target goes away.  This will avoid spurious UI-level errors to staff.

Signed-off-by: Mike Rylander <mrylander@gmail.com>