docs: admin-guide/blockdev: Remove digraph of node-states

While node-states-8.dot has two digraphs, the dot(1) command can
not properly handle multiple graphs in a DOT file and the
kernel-doc page at

    https://www.kernel.org/doc/html/latest/admin-guide/blockdev/drbd/figures.html

fails to render the graphs.

It turned out that the digraph of node_states can be removed.

Quote from Joel's reflection:

    On reflection, the digraph node_states can be removed entirely.
    It is too basic to contain any useful information. In addition
    it references "ioctl_set_state". The ioctl configuration
    interface for DRBD has long been removed. In fact, it was never
    in the upstream version of DRBD.

Remove node_states and rename the DOT file peer_states-8.dot.

Suggested-by: Joel Colledge <joel.colledge@linbit.com>
Acked-by: Joel Colledge <joel.colledge@linbit.com>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Link: https://lore.kernel.org/r/7df04f45-8746-e666-1a9d-a998f1ab1f91@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/blockdev/drbd/figures.rst

@@ -25,6 +25,6 @@ Sub graphs of DRBD's state transitions
     :alt:   disk-states-8.dot
     :align: center
 
-.. kernel-figure:: node-states-8.dot
-    :alt:   node-states-8.dot
+.. kernel-figure:: peer-states-8.dot
+    :alt:   peer-states-8.dot
     :align: center

Documentation/admin-guide/blockdev/drbd/peer-states-8.dot

@@ -1,8 +1,3 @@
-digraph node_states {
-	Secondary -> Primary           [ label = "ioctl_set_state()" ]
-	Primary   -> Secondary 	       [ label = "ioctl_set_state()" ]
-}
-
 digraph peer_states {
 	Secondary -> Primary           [ label = "recv state packet" ]
 	Primary   -> Secondary 	       [ label = "recv state packet" ]