Remove CQM._discrete private attribute
Remove CQM._discrete private attribute

...and instead just look through the existing discrete constraints.
This changes the complexity from O(1) to O(# of discrete constraints)
but that number will generally be low and this way we're not keeping
duplicated information that needs to be tracked.

If we later find this to be a performance bottleneck we can restore
it.

dwavesystems/dimod,dwavesystems/dimod