[SPARK-9050] [SQL] Remove unused newOrdering argument from Exchange (cleanup after SPARK-8317)

SPARK-8317 changed the SQL Exchange operator so that it no longer pushed sorting into Spark's shuffle layer, a change which allowed more efficient SQL-specific sorters to be used.

This patch performs some leftover cleanup based on those changes:

- Exchange's constructor should no longer accept a `newOrdering` since it's no longer used and no longer works as expected.
- `addOperatorsIfNecessary` looked at shuffle input's output ordering to decide whether to sort, but this is the wrong node to be examining: it needs to look at whether the post-shuffle node has the right ordering, since shuffling will not preserve row orderings.  Thanks to davies for spotting this.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7407 from JoshRosen/SPARK-9050 and squashes the following commits:

e70be50 [Josh Rosen] No need to wrap line
e866494 [Josh Rosen] Refactor addOperatorsIfNecessary to make code clearer
2e467da [Josh Rosen] Remove `newOrdering` from Exchange.