Remove ChurnBuster integration

Integrating solidus_subscriptions with ChurnBuster seemed like a good idea, but the reality is that it's impossible to make the two work well together: ChurnBuster works well when it's integrated with a natively-supported platform such as Shopify/ReCharge. When that's the case, ChurnBuster can manage the subscriptions directly (e.g., by cancelling them after sending the churn recovery email sequence).

When ChurnBuster is integrated with Solidus and Solidus initiates ChurnBuster campaigns, the timing of emails going out in ChurnBuster and Solidus taking actions such as cancelling the subscription would need to be perfectly synchronized (i.e. Solidus would need to cancel the subscription right when ChurnBuster sends the "subscription cancelled" email). This is impossible to achieve in the real world, which leads to a subpar UX.

Churn recovery with solidus_subscriptions is better done with other services or other third-party tools, but ChurnBuster is definitely not a good option.

My suggestion is to simply remove the integration in the next major version with no deprecation in between.

Start by locating the ChurnBuster integration and all references to it in solidus_subscriptions. Remove the integration as part of the next major version, with no intermediate deprecation, and verify that the remaining subscription functionality still works without it.