Replace invalid field `gas_used` in expected_receipts in 7778 test

[danceratopz]

I think this is currently only a no-op. It only effects fill verification; the fixtures remain unchanged.

        expected_receipt={
            "cumulative_gas_used": refund_tx_gas_used,
        },

The gas_used field does not exist in the transition tool:

execution-specs/src/ethereum/forks/amsterdam/blocks.py

Lines 347 to 382 in 9d88e40

	@slotted_freezable
	@dataclass
	class Receipt:
	"""
	Result of a transaction execution. Receipts are included in the receipts
	trie.
	"""
	succeeded: bool
	"""
	Whether the transaction execution was successful.
	"""
	cumulative_gas_used: Uint
	"""
	Total gas used in the block up to and including this transaction.
	This is the gas used after refunds, paid by the user.
	"""
	bloom: Bloom
	"""
	Bloom filter for logs generated by this transaction. This is a 2048-byte
	bit array that allows for efficient filtering of logs.
	"""
	logs: Tuple[Log, ...]
	"""
	A tuple of logs generated by this transaction. Each log contains the
	address of the contract that emitted it, a tuple of topics, and the data
	payload.
	"""
	def encode_receipt(tx: Transaction, receipt: Receipt) -> Bytes | Receipt:
	r"""
	Encodes a transaction receipt based on the transaction type.

This requires a follow-up issue to detect in the helper. I will follow-up.

Originally posted by @danceratopz in #2840 (comment)

As I understand, the check is redundant as gas_used does not exist in t8n; it uses cumulative_gas_used (L360 above). This is a sanity check (it is not propagated to the fixtures), but is incorrectly verified in the helpers. We can fix the field here, but add a follow-up to improve the helper.