This keeps the `OrderBalance` abstraction but removes the old code now
that the feature is enabled for all users in all instances and there are
no bugs reported. It's become dead code.
This makes it possible to deploy it without releasing it to users since
the toggle is not enabled for anyone.
It aims to make the balance calculation consistent across pages.
We only care about non-cart orders and skipping carts, saves PostgreSQL
query planner to go through thousands of records in production use cases
(my food hub).
We go from
```sql
-> Index Scan using index_spree_orders_on_customer_id on spree_orders (cost=0.42..12049.45 rows=152002 width=15) (actual time=0.015..11.703 rows=13867 loops=1)
```
to
```sql
-> Index Scan using index_spree_orders_on_customer_id on spree_orders (cost=0.42..12429.46 rows=10802 width=15) (actual time=0.025..17.705 rows=9954 loops=1)
```
As is, `payment_total` is only increased after successfully processing
a payment and never updated. This inconsistency breaks
`CustomerWithBalance` which relies on it.
Needless to say that if we keep this denormalized column, we better make
it consistent. I investigated current Spree's master branch (709e686cc0)
and they also realized it was broken. Now `Payment` runs the following
from the `after_save` `update_order` callback.
```rb
order.updater.update_payment_total if completed? || void?
```
I also took the chance to rearrange tests a bit.
