remove standard gem (revert to plain rubocop)

[ndushay]

remove the standard gem in favor of plain rubocop.

per discussion at dev planning https://docs.google.com/document/d/1gzKgbqnlCc_GctBBpoYTvZl22mNZflsbqzyz4xXJlho/edit#heading=h.hmax9277viw

It probably doesn't matter for a gem, but take a look at argo/issues/4214 for what goes in .rubocop.yml as:

1. we may want to explicitly have some non-default settings for rules (e.g. line length)
2. if there is a lot of stuff in logs or tmp or vendor or whatever, running rubocop can be much slower since it looks at every directory.