Remove checkout step (#3519)

Summary:
According to [this](https://circleci.com/docs/2.0/configuration-reference/#checkout), using the checkout step uses SSH but we're fine with HTTPS.

Pull Request resolved: https://github.com/facebook/flipper/pull/3519

Reviewed By: lblasa

Differential Revision: D34818577

Pulled By: passy

fbshipit-source-id: 1f34ab1d183081125607e6d2020bc7a1c7c34685

.circleci/config.yml

@@ -13,7 +13,6 @@ jobs:
   snapshot:
     executor: default-executor
     steps:
-      - checkout
       - run:
           name: install retry
           command: scripts/install-retry.sh