The default `bitraverse` implementation should be removed.

The default `bitraverse` is
`bitraverse f g = bisequenceA . bimap f g`.

But `bisequenceA` is an alias of `bisequence`, which in turn is `bitraverse id id`. Neither `bisequence` nor `bisequenceA` is a Bitraversable method, so they can't have alternative definitions.

Sorry if this has already been mentioned.

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