cumulus-pov-recovery: check pov_hash instead of reencoding data (#2287)

Collators were previously reencoding the available data and checking the
erasure root.
Replace that with just checking the PoV hash, which consumes much less
CPU and takes less time.

We also don't need to check the `PersistedValidationData` hash, as
collators don't use it.

Reason:
https://github.com/paritytech/polkadot-sdk/issues/575#issuecomment-1806572230

After systematic chunks recovery is merged, collators will no longer do
any reed-solomon encoding/decoding, which has proven to be a great CPU
consumer.

Signed-off-by: alindima <[email protected]>