dfrac: do not track discarded fraction

While porting gmap_auth to dfrac, I realized one annoyance with that construction: to state that something is discarded requires saying what fraction got (at least) discarded.

This MR solves that problem by removing the fraction from DfracDiscarded. That elements now represents the idea that "some fraction has been discarded", but it is not relevant which fraction. DfracOwn q ⋅ DfracDiscardrd is valid if and only if q < 1.