Bug 29741: (follow-up) Make DELETE /patrons use the new validation method

Bug 29741: (follow-up) Make DELETE /patrons use the new validation method

This patch adapts the route so it uses the newly introduced
Koha::Patron->safe_to_delete method.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests still pass!
4. Sign off :-D

Note: There's a trivial behavior change, in which the 'anonymous patron'
use case is caugh eariler than the ->delete call. I left the exception
catch block just in case, who knows :-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>