Tidy RebalanceTreasury and SimulatedBackend

[blukat29]

Proposed changes

• Remove unused getBurnAmountMagma function
• Relocate the treasury rebalance memo logging site from system.RebalanceTreasury to engine.Finalize
• SimulatedBackend.SuggestGasPrice returns lastBaseFee*2 or UnitPrice depending on fork level.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

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• I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
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• Any dependent changes have been merged and published in downstream modules

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Further comments

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