Remove project level rubocop.yml (#1531)

Rubocop brings with it a certain amount of maintenence burden and
keeping project level config files means they're always somewhat out of date.
Additionally, we already have [thoughtbot/guides][one] which is the
source of truth.

This removes that to see if we get the right outcome.

[one]: https://github.com/thoughtbot/guides/blob/master/style/ruby/.rubocop.yml