Use rapidsai CODE_OF_CONDUCT.md (#1159)

This repo's `CODE_OF_CONDUCT.md` is superseded by an organization-wide policy: rapidsai/.github#3

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #1159