Bug 29018: Make DELETE /patrons/:patron_id check things

Bug 29018: Make DELETE /patrons/:patron_id check things

When the route was implemented, the checks were overlooked. This patch
adds checks for:
- Guarantees
- Debts
- Current checkouts

Any of those will block deletion, as it should.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons.t
=> FAIL: Tests fail, the route misses checks
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass! The three conditions prevent deletion!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>