Remove misleading assume example.

[abeakkas]

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Given snippet doesn't find any satisfying examples anymore. Not sure if it can be fixed through tweaking some settings but it is misleading in its current state. I don't know what changed since the original documentation was added, maybe @DRMacIver can comment.

See the relevant PR: https://github.com/HypothesisWorks/hypothesis/pull/2600 which was mistakenly reverted in: https://github.com/HypothesisWorks/hypothesis/pull/3238

[DRMacIver]

Huh it's hard to see how this example would ever have passed in modern Hypothesis. I wonder how old it is - I think some of the early versions had features that might have allowed it to pass, but I think it's probably been broken since 3.0 came out. Thanks for the catch!