Temporarily duplicated MAX_POV_SIZE should be replaced with polkadot_primitives::MAX_POV_SIZE after SDK release

[muharem]

The current MAX_POV_SIZE value for async backing is temporarily duplicated (#718) from the updated polkadot_primitives::MAX_POV_SIZE constant.

Once paritytech/polkadot-sdk#5884 is included in a released version of the Polkadot SDK, this value should be replaced with a direct reference to the constant in polkadot_primitives to ensure consistency.