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.co...