Stop implicitly loading play_player data.

This was once useful when plays and play_players were
irrevocably tied together. But now that play has its
own aggregate data, we no longer need this.

nfldb/types.py

@@ -1744,12 +1744,6 @@ def __str__(self):
  def __getattr__(self, k):
  if k in Play.__slots__:
  return 0
- if k in PlayPlayer.__slots__:
- for pp in self.play_players:
- v = getattr(pp, k)
- if v != 0:
- return v
- return 0
  raise AttributeError(k)