ENH: Remove dependency on benchmark for trading day calendar.

Instead of the benchmarks' index, use the trading calendar to
populate the environment's trading days.

Remove `extra_date` field, since unlike the benchmarks list,
the trading calendar can generate future dates, so dates for
current day trading do not need to be appended.

Motivations:
- The source for the open and close/early close calendar and the
  trading day calendar is now the same, which should help prevent
  potential issues due to misalignment.
- Allows configurations where the benchmark is provided as a
  generator based data source to need to supply a second benchmark
  list just to populate dates.