Remove PRODUCT_SHIPPING_API_LEVEL in mainline_system

Let this be defined in the leaves instead. As is it frequently causes
problems where the product inheriting this also inherit another value
of PRODUCT_SHIPPING_API_LEVEL, leading to invalid values unless
overridden again.

Bug: 80410283
Test: lunch mainline_arm64; m nothing
Test: lunch mainline_system_arm64; m nothing
Change-Id: Id08b6cc0cd8815404e548c521c9e40706ba8ca16