[Inventory] Remove StockExtension class and interface

[sivaschenko]

Description

Even though extension attributes PHP classes/interfaces were not supposed to be created manually, we have several such occurrences.
It is preferable to remove these classes/interfaces because they block extensibility of the base interfaces.

Details

Remove Magento\InventoryApi\Api\Data\StockExtension and Magento\InventoryApi\Api\Data\StockExtensionInterface in future-develop branch

[m2-assistant]

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[ProkopovVitaliy]

@magento i am working on this

[sdouma]

@magento-engcom-team can we get movement on this? This breaks any extending the stock interface and has been a pain