tests: secure_storage: remove an integration platform

nrf54l15dk/nrf54l15/cpuapp already covers the non-emulated case as
part of the integration_platforms.
Having ophelia4ev/nrf54l15/cpuapp does not bring any additional value
and increases CI load, so remove it.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>