Allow "email" param by itself for CloudFlare::user_lookup

- Remove user validation conditional in "user_lookup" sub.
  It doesn't seem necessary to validate that the specified "user"
  exists on the server.
- "user" param is set by __load_data_file() so there was no case in
  which the `else` clause after `if
  $cf_global_data->{"cf_user_tokens"}->{ $OPTS{"user"} }` was
  actually reached, preventing user lookup by "cloudflare_email"
  param.