ima: Remove EXPERIMENTAL from Kconfig

Remove the EXPERIMENTAL from the
IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY Kconfig
now that digitalSignature usage enforcement is set.

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
link: https://lore.kernel.org/all/20230508220708.2888510-4-eric.snowberg@oracle.com/


Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>