Improved account deletion

- Removed old "WordPress.com" credential deletion. We've had accounts
  for a while
- Don't cancel/reset the API object if it's not initialized
Moved wp.com cookie deletion to AccountService

WordPressComApi should be only about REST.
AccountService should be handling self-hosted account removal and
clearing cookies for those as well, but that's another issue
Merge pull request #1858 from wordpress-mobile/issue/1842-keychain-sign-out

#1842 keychain sign out issues