Bug 28907: REST - Remove allow-owner from public checkout availability route

Bug 28907: REST - Remove allow-owner from public checkout availability route

To test:
1. prove t/db_dependent/api/v1/checkouts.t
2. Apply patch
3. prove t/db_dependent/api/v1/checkouts.t

Observe success in both cases.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
(cherry picked from commit e6cd5ae59f5f075b89b7fc7ee5a06e8ecfd9d3eb)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>