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
> 1. Should we remove the similar method in go-syncuser in favor of each plugin having its own method, or should we consolidate around the method there?

i think the similar method in `go-syncuser` isn't actually being used. it was added when I thought we were going to have one unified sync button, but we ended up going the other way. i would remove the one in `go-syncuser` once I confirm that it's indeed not being used by any one.