Remove handling of extra argument to ``__array__``
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A code path and test have been in the code since NumPy 0.4 for a two-argument
variant of ``__array__(dtype=None, context=None)``. It was activated when
calling ``ufunc(op)`` or ``ufunc.reduce(op)`` if ``op.__array__`` existed.
However that variant is not documented, and it is not clear what the intention
was for its use. It has been removed.