Delete obsolete docs on GADT interacton with TypeApplications
Even since ef26182e2014b0a2a029ae466a4b121bf235e4e4, this section of
the users' guide is wrong, as there are no longer special rules for
the order of type variables in GADT constructors' type signatures
vis-à-vis visible type application. As a result, this section can
simply be deleted, as there is no longer anything interesting to say
about the topic.