pull requests: rm "cron" settings

pull requests: rm "cron" settings

From the Jenkins GitHub Pull Requests plugin docs on "useGitHubHooks":

  Checking this option will disable regular polling (cron) for changes
  in GitHub and will try to create a GitHub hook. Creating a GitHub hook
  requires that the user which is specified in the GitHub Pull Request
  Builder configuration has admin rights to the specified repository.

Based on the documentation above, since our PR jobs have "github-hooks:
true", the "cron" values have no effect and can be removed.