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.