Bad default bitraverse

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. Sent from my phone with K-9 Mail.