Remove CircleCI

[mitchellhenke]

With #6614 live, we should be able to drop CircleCI if we want 🙂

[aduth]

There's a handful of other references to CircleCI, should we clean those up as well?

$ ag circleci -i --ignore-dir=node_modules
config/environments/test.rb
39:  # Disable lograge when computing coverage and not in CircleCI, where lograge is required.

spec/simplecov_helper.rb
64:      # CircleCI uses JSON formatting for CodeClimate

README.md
4:[![Build Status](https://circleci.com/gh/18F/identity-idp.svg?style=svg)](https://circleci.com/gh/18F/identity-idp)

$ ag circle_ci -i --ignore-dir=node_modules
spec/simplecov_helper.rb
63:    elsif ENV['CIRCLE_CI']

[mitchellhenke]

There's a handful of other references to CircleCI, should we clean those up as well?

$ ag circleci -i --ignore-dir=node_modules
config/environments/test.rb
39:  # Disable lograge when computing coverage and not in CircleCI, where lograge is required.

spec/simplecov_helper.rb
64:      # CircleCI uses JSON formatting for CodeClimate

README.md
4:[![Build Status](https://circleci.com/gh/18F/identity-idp.svg?style=svg)](https://circleci.com/gh/18F/identity-idp)

$ ag circle_ci -i --ignore-dir=node_modules
spec/simplecov_helper.rb
63:    elsif ENV['CIRCLE_CI']

thank you for catching, deleted these