Functionality using pickle should be removed

[brittonsmith]

A number of issues (e.g., Issue #1836, Issue #1898) have come up related to pickling of yt data objects or saving yt data objects using pickle-related functions. Much of this relies on the parameter_file_storage functionality that is also no longer functional. However, the ytdata frontend and the save_as_dataset machinery have made pickle-related stuff mostly redundant. Given this, we should remove the picke-related stuff in favor of ytdata-related functionality.

[matthewturk]

Seems reasonable.