Feature/remove osffiles

Imported from https://github.com/CenterForOpenScience/osf/pull/1241

# Purpose
Remove unused code related to original OSF Files

# Changes
* Remove `osffiles` add-on
* Remove file-related methods from `Node`
* Remove unused dependencies, imports

# Side effects
* Breaks migration tests for OSF Storage (which is why this should not be merged until after the migration is complete)