fs: dlm: remove unused processed_nodes

The variable processed_nodes is not being used by commit 1696c75f
("fs: dlm: add send ack threshold and append acks to msgs"). This patch
removes the leftover of this commit.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>