MAINT: Remove unused calculations of max_leverage, et al.

In the performance period the max_leverage, max_capital_used,
cumulative_capital_used were calculated but not used.

At least one of those calculations, max_leverage, was causing a
divide by zero error.
Instead of papering over that error, the entire calculation was
a bit suspect so removing, with possibility of adding it back in
later with handling the case (or raising appropriate errors) when
the algorithm has little cash on hand.