Remove unused variable assignment chap11 nb - #69

Applied by hand to soln/ and notebooks/ rather than merged, because the
PRs edit notebooks/, which is regenerated from soln/. Thanks to
@gwtaylor for all of these.

- #59: typos in appA and chap02 (the nodes_iter change is not applied;
  nodes_iter was removed in NetworkX 2)
- #62: typo in chap08
- #63: remove the duplicate introduction of Methuselahs in chap06
- #64: "Facecook" in chap04
- #68: typos and the stale Simulation docstring in chap11 (also fixed
  in chap12, which has the same docstring)
- #69, #70: remove an unused variable in Simulation.step in chap11 and
  chap12; the chap12 score range is 0-5, not 0-60

#60 and #61 were already fixed. #65 is not applied: the code in that
section counts red and blue neighbors, so "non-empty" is correct.

Also fix "some initial conditional run" in chap06.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>