Only reload shipment items on item adjustment

The original issue fixed by https://github.com/spree/spree/commit/a4b209c7288198124a735d99203102d86be6a935
only affected shipment calculations. Removing the reload for LineItems
allows them, and more important their descendants to be eager loaded.

Also it less frequently nukes lazy load caches, resulting in a small (but under load measurable) performance improvement. It has no semantic differences.

This is part of a series of commits that leads to a more fundamental performance improvement.

I'd like to see this backported at least to `2-3-stable`.