Bug 30727: Avoid holds queue updates multiple times on BatchDeleteBiblio

Bug 30727: Avoid holds queue updates multiple times on BatchDeleteBiblio

This patch makes the background job only enqueue holds queue updates
once per biblio, when appropriate.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/BackgroundJobs/BatchDeleteBiblio.t
=> FAIL: It enqueues 3 times for a bib!
3. Apply this patch
4. Repeat 2
=> SUCCESS: It only enqueues once!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>