MAINTAINERS: remove David Safford as maintainer for encrypted+trusted keys

Emails to David's listed email address bounce, and in the commit log
there's no activity from him within the last 5 years.

Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: David Safford <safford@us.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>