fix(tests): Revert MIP-11 shim

[pdobacz]

reverts part of #30 so that the MONAD_NEXT release works again

Greptile Summary

This PR reverts the Monad Next priority-fee test shim. The main changes are:

• Restores priority fee calculation in monad_next transaction processing.
• Credits transaction priority fees to the block coinbase again.
• Removes the Monad Next-specific expected reward branch from the reserve-balance test.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
src/ethereum/forks/monad_next/fork.py	Restores coinbase priority-fee crediting in process_transaction, matching sibling fork behavior.
tests/monad_eight/reserve_balance/test_transfers.py	Updates the transaction-fee reward expectation and removes the now-unused Monad Next import.

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