CreateGithubWebhookWorker attempts to create too many hooks

https://sentry.gitlab.net/gitlab/gitlabcom/issues/674432/

Octokit::UnprocessableEntity: POST https://api.github.com/repos/bejabri/c0da/hooks: 422 - Validation Failed
Error summary:
  resource: Hook
  code: custom
  message: The "push" event cannot have more than 20 hooks // See: https://developer.github.com/v3/repos/hooks/#create-a-hook
  octokit/response/raise_error.rb:16:in `on_complete'
    raise error
  faraday/response.rb:9:in `block in call'
    on_complete(environment)
  faraday/response.rb:61:in `on_complete'
    yield(env)
  faraday/response.rb:8:in `call'
    @app.call(env).on_complete do |environment|
  octokit/middleware/follow_redirects.rb:73:in `perform_with_redirection'
    response = @app.call(env)
...
(59 additional frame(s) were not displayed)

POST https://api.github.com/repos/bejabri/c0da/hooks: 422 - Validation Failed
Error summary:
  resource: Hook
  code: custom
  message: The "push" event cannot have more than 20 hooks // See: https://developer.github.com/v3/repos/hooks/#create-a-hook