refactor!: remove useless fields in BlockHeader

[KaoImin]

What this PR does / why we need it?

This PR remove these three fields in Header:

1. difficulty
2. nonce
3. mixed_hash

They are all mining related and useless to Axon. The rpc will return the default value.

What is the impact of this PR?

⚠️This is a breaking change.

CI Settings

CI Usage

Tip: Check the CI you want to run below, and then comment /run-ci.

CI Switch

• Cargo Clippy
• Coverage Test
• E2E Tests
• Code Format
• Unit Tests
• Web3 Compatible Tests
• OCT 1-5 And 12-15
• OCT 6-10
• OCT 11
• OCT 16-19
• v3 Core Tests

CI Description

CI Name	Description
Chaos CI	Test the liveness and robustness of Axon under terrible network condition
Cargo Clippy	Run cargo clippy --all --all-targets --all-features
Coverage Test	Get the unit test coverage report
E2E Test	Run end-to-end test to check interfaces
Code Format	Run cargo +nightly fmt --all -- --check and cargo sort -gwc
Web3 Compatible Test	Test the Web3 compatibility of Axon
v3 Core Test	Run the compatibility tests provided by Uniswap V3
OCT 1-5 | 6-10 | 11 | 12-15 | 16-19	Run the compatibility tests provided by OpenZeppelin

[Flouse]

Is it necessary to update the test data?

axon/tests/data/genesis.json

Lines 12 to 19 in db0c608

	"difficulty": "0x0",
	"timestamp": 1679656015,
	"number": 0,
	"gas_used": "0x0",
	"gas_limit": "0x0",
	"extra_data": [],
	"mixed_hash": null,
	"nonce": "0x0000000000000000",

[KaoImin]

/run-ci

[axon-bot]

CI tests run on commit:

• commit id: fbb4370

CI test list:

• Code Format
• Cargo Clippy
• E2E Tests
• OCT 1-5 And 12-15
• OCT 6-10
• OCT 11
• OCT 16-19
• Unit Tests
• v3 Core Tests
• Web3 Compatible Tests

Please check ci test results later.