… #13780

`CIVICRM_FLEXMAILER_HACK_LISTENERS` and `registerListeners()` function were
used in older versions to wire-up Flexmailer's dispatchers.  This is now
accomplished via public API, `hook_civicrm_container` - seems to work, no
reason to go back.

Note: I was confused by two things:

1. The previous calls didn't look like they should work.

2. Somehow, the API override still worked - even though there was no obvious callpath to `addSubscriberService()`
   It turns out that `registerApiProvider()` handles registration with the dispatcher, so `addSubscriberService()`
   isn't actually used.

In core's #17380, we ultimately preserved `addListenerService()`, so this doesn't have to be removed.