remove user certificate if not found on the server
If the user is not found by cert lookup when the user is already
cached, two things may happen:
1) cert was removed from the user object
2) user was removed
Instead of issuing another cert lookup we will just remove cert
attribute from the cache not touching the expiration timestamp so
the user may be updated later when needed.
Resolves:
https://fedorahosted.org/sssd/ticket/2934
Reviewed-by: Sumit Bose <sbose@redhat.com>