Bug 29765: Adjust DELETE /patrons/:patron_id controller

Bug 29765: Adjust DELETE /patrons/:patron_id controller

This patch adjusts the only current place using the
Koha::Patron->safe_to_delete method to the new return value. There
should be no behavior change

To test:
1. Apply the previous patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons.t
=> FAIL: Tests fail, because of the change, the controller is broken
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass! No test tweak required, no behavior change
5. Sign off :-D

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>