fs: dlm: remove unused struct 'dlm_processed_nodes'

The last use of 'dlm_processed_nodes' was removed in
commit 1696c75f ("fs: dlm: add send ack threshold and append acks to
msgs").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: David Teigland <teigland@redhat.com>