Remove redundant empty slot count check
Remove redundant empty slot count check

USE_PSA_DONE() already checks that there are no used key slots.

The call to TEST_ASSERT() wouldn't have worked properly on failure anyway,
since it would jump back to the exit label.

Signed-off-by: Andrzej Kurek <f4c472ffe6ee3dcd17a8925cef48ace45ab528e9@arm.com>

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls