chore: remove eth_hashrate and eth_mining json rpc methods

[randy-cro]

Rationale

These methods are irrelevant as our chain is POS and not POW.

References

https://github.com/cosmos/evm/pull/456/files

Closes: #XXX

Description

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• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
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[thomas-nguy]

may need to update the changelog

[XinyuCRO]

there are also MinerNamespace declared in api, should we remove it as well?