Respond to deletion of a node revision.
This hook is invoked from node_revision_delete() after the revision has been removed from the node_revision table, and before field_attach_delete_revision() is called.
$node: The node revision (node object) that is being deleted.
function hook_node_revision_delete($node) {
db_delete('mytable')
->condition('vid', $node->vid)
->execute();
}