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.