Clean up state pkg's getter and setter

[terencechain]

What type of PR is this?

Cleanup

What does this PR do? Why is it needed?
Before we use interface for state pkg's getter and setter, I want to do one round of clean up. Changes:

• HasInnerState() can be private hasInnerState()
• Getter's GenesisUnixTime and ParentRoot are not used. They dont seem to have much use cases so deleting them
• Setter's SetValidatorIndexByPubkey is not used. Deleting it
• Validator read only getter's CopyValidator() is not used. Deleting it
• Move validator read only getter's to validator_getter.go to stay lean

Which issues(s) does this PR fix?

N/A

Other notes for review

N/A

[codecov]

Codecov Report

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