integrity: eliminate unnecessary "Problem loading X.509 certificate" msg

Currently when the kernel fails to add a cert to the .machine keyring,
it will throw an error immediately in the function integrity_add_key.

Since the kernel will try adding to the .platform keyring next or throw
an error (in the caller of integrity_add_key i.e. add_to_machine_keyring),
so there is no need to throw an error immediately in integrity_add_key.

Reported-by:  <itrymybest80@protonmail.com>
Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2239331


Fixes: d1996776 ("integrity: Introduce a Linux keyring called machine")
Reviewed-by: Eric Snowberg <eric.snowberg@oracle.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>