Shipping Calculators still calculate for cancelled items #1837

Steps to reproduce

1. Complete order with many items and flexirate shipping calculator
2. Cancel items in shipment
3. Adjustment created does not take into account shipping cost
4. Cost on shipping was not changed even though less items will be shipped

Expected behavior

After an item is cancelled the calculator should only calculate cost based on what is actually going to ship.

Actual behavior

Shipping calculators are calculating cost on all items even if they are cancelled:

• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/flexi_rate.rb#L12
• https://github.com/solidusio/solidus/blob/master/core/app/models/spree/calculator/shipping/flexi_rate.rb#L12

• solidus/core/app/models/spree/calculator/shipping/per_item.rb

  Line 14 in 054e4bd

  compute_from_quantity(package.contents.sum(&:quantity))

• etc.

System configuration

Solidus Version: 2.1 (seems to be in all)

Extensions in use: core, frontend, backend