Bug 30536: Remove validation overhead

Bug 30536: Remove validation overhead

With the introduction of proper validation of collection headers in
OpenAPI we no longer need to do our own validation here.

This patch removes the early validation of x-koha-embed headers in
preference to letting OpenAPI do it once and only once instead.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>