Fix uncaptured amount calculation

Under certain circumstances, Payment's uncaptured_amount won't always have the correct value in the database. After discussing in #spree-dev IRC, it seems like there is no reason we shouldn't just calculate this value on the fly instead of trying to keep the database in sync.

* Remove the callback attempting to save the uncaptured amount to the database
* Add a new method to calculate the uncaptured amount
* Remove database column
* Specs