Refactor to use 'woocommerce_subscription_is_manual' filter

[thenbrent]

To be able to fully test subscription payment functionality, like the automatic retry system, on a staging site, we need to be able to override a subscriptions is_manual() return value. This will be possible once https://github.com/Prospress/woocommerce-subscriptions/pull/3102 is merged, and that can be used in this plugin instead of the current approach.