security,selinux: remove security_add_mnt_opt()

Its last user has been removed in commit f2aedb71 ("NFS: Add
fs_context support.").

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>