Remove only_free param to Pool.get()

This is unused. I think I added it before iter_free(), i.e. empty()
was going to be:

  for item in self.get(only_free=True):
      item.close()

rather than:

  for item in self.iter_free():
      item.close()

Change-Id: Ie6d0de3b7453ee036412db10f37d26f04de73cf7
Merge "Remove only_free param to Pool.get()"