more debug output plus a couple of cleanup fixes

* removed an unused object var $debug
* more slogging when `go-syncuser()->debug()` is `TRUE`
* removed some extraneous calls to [`subscribe()`](https://github.com/GigaOM/go-mailchimp/blob/4edb240d8584a5c6d6e9a2d36436d1284b0ff12f/components/class-go-mailchimp-api.php#L331) and [`unsubscribe()`](https://github.com/GigaOM/go-mailchimp/blob/4edb240d8584a5c6d6e9a2d36436d1284b0ff12f/components/class-go-mailchimp-api.php#L481) due to mismatched array format passed in by [`user_sync_ajax()`](https://github.com/GigaOM/go-mailchimp/blob/4edb240d8584a5c6d6e9a2d36436d1284b0ff12f/components/class-go-mailchimp-admin.php#L184)

related to https://github.com/GigaOM/legacy-pro/issues/4020
[Elsewhere](https://github.com/GigaOM/go-mailchimp/pull/8/files#diff-27fc7662f48f05c63ee83bfb079e0806R224) in this diff i see we `if ( go_syncuser()->debug() )` a lot, but not in this particular change, nor back up [here](https://github.com/GigaOM/go-mailchimp/pull/8/files#diff-27fc7662f48f05c63ee83bfb079e0806R177)?