Fix ReadOnlyError when destroying StockItem

StockMovement is a readonly object so we can't delete them. And by that
logic I believe we shouldn't clear stock items from the database either
otherwise we will have stock movements associated with non existing
stock items

Fixes #3727
Fixes #3723
Revert "fix the ActiveRecord::ReadOnlyRecord exception when the @product is null when update the variant"

This reverts commit 8660b46a546418d91ffec02124303e13b1a7a6a8.
Revert "add spec to proof issue with variant update without product permalink."

This reverts commit f39a2f6a8c9435595245c47b9b2fb86a00db9f31.
Prep for 2.0.5 release