[docs] [issue18696] In unittest.TestCase.longMessage doc remove a redundant sentence

[R. David Murray]

R. David Murray added the comment:

I think the text is OK as it stands.  The first sentence is a technical description of the result of setting the attribute, the last sentence is a conceptual discussion of why you would want to set the attribute.

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nosy: +r.david.murray

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<http://bugs.python.org/issue18696>
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