Remove post processing methods

[jkeifer]

With #181 landing, we have the problematic situation that the post processing methods could be doing item modifications that would not be captured in the uploaded canonical items. A discussion between @matthewhanson, @ping-e84, and @jkeifer resulted in the decision to remove the post processing methods, and auditing the current extension sorting to see if that needs to be retained, and if so if it should be integrated into the upload method.

This is a breaking change, but we expect few if any stac task instances are actually overriding the post processing method(s).