[docs] simplify Benders tutorial with copy of state variables

[odow]

There is almost never a case in which the dual form is a better choice. The x_copy trick is far simpler, with minimal room for mistakes. And all for the low, low price of a few extra variables that get removed in presolve.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.90%. Comparing base (4219ccb) to head (7a58359).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3832   +/-   ##
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  Coverage   97.90%   97.90%           
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  Files          44       44           
  Lines        6019     6019           
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  Hits         5893     5893           
  Misses        126      126           

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