Move Gitlab::Shell authorized_keys_enabled? nil workaround to EE module with note to remove

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Why

This workaround is not needed in CE, and isn't needed in EE once the background migrations have completed. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16014#note_53529819

What

Remove the following from lib/gitlab/shell.rb, or move it to an EE module with a note to remove:

      # Return true if nil to ensure the authorized_keys methods work while
      # fixing the authorized_keys file during migration.
      return true if Gitlab::CurrentSettings.current_application_settings.authorized_keys_enabled.nil?