Remove incorrect usage of additionalProperties

[nflaig]

The usage of additionalProperties: false here is incorrect as each sub schema is evaluated independently, so what ends up happening is that when the second schema is evaluated which only has transactions_root as property, it will complain that there are additional properties if we pass a ExecutionPayloadHeader value since it has more properties like parent_hash.

This is the error when running spec tests in Lodestar

/body/message/body/execution_payload_header - must NOT have additional properties

And the more detailed error

{
  instancePath: "/body/message/body/execution_payload_header",
  schemaPath: "#/properties/body/anyOf/0/properties/message/allOf/1/properties/body/allOf/1/properties/execution_payload_header/allOf/1/additionalProperties",
  keyword: "additionalProperties",
  params: {
    additionalProperty: "parent_hash",
  },
  message: "must NOT have additional properties",
}

It also seems like we only use additionalProperties: false in the bellatrix types.

eg. capella types don't use it

beacon-APIs/types/capella/execution_payload.yaml

Lines 49 to 53 in 5815ac5

	allOf:
	- $ref: '#/Capella/ExecutionPayloadCommon'
	- type: object
	required: [transactions_root, withdrawals_root]
	properties: