Stop passing line_item to compute unit cancelation adjustment's amount

[BenMorganIO]

Summary

Simple refactoring for the Spree::UnitCancel. There's no need to do inventory_unit.line_item and then pass the line item through everywhere when we can get it through a has_one. I also changed weighted_amount to accept an arbitrary amount instead of a line item object.

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[kennyadsl]

compute_amount is a public method. We have two ways of handling the change of the signature of a public method:

1. Rename the method and deprecate compute_amount, delegating its usage to the new method.
2. Keep compute_amount with the same signature but make the argument optional and deprecate its usage.

I think it works both ways, up to you!