KEYS: Remove KEY_FLAG_TRUSTED and KEY_ALLOC_TRUSTED

Remove KEY_FLAG_TRUSTED and KEY_ALLOC_TRUSTED as they're no longer
meaningful.  Also we can drop the trusted flag from the preparse structure.

Given this, we no longer need to pass the key flags through to
restrict_link().

Further, we can now get rid of keyring_restrict_trusted_only() also.

Signed-off-by: David Howells <dhowells@redhat.com>