Cleaning up unused functions and variables and add more documentation

[lumtis]

emissions module has some unused variables and methods that makes understanding the code complicated. Example not used:

• https://github.com/zeta-chain/node/blob/6c0e0bba0e1c03e916cbf05667b22f44b501f555/x/emissions/types/keys.go#L28
• https://github.com/zeta-chain/node/blob/6c0e0bba0e1c03e916cbf05667b22f44b501f555/x/emissions/keeper/block_rewards_components.go#L68

We should remove all these unused components.

We should also add more documentation for the variables and functions. Example:

• How this value has been calculated: https://github.com/zeta-chain/node/blob/6c0e0bba0e1c03e916cbf05667b22f44b501f555/x/emissions/types/keys.go#L55
• What does this function do: https://github.com/zeta-chain/node/blob/6c0e0bba0e1c03e916cbf05667b22f44b501f555/x/emissions/keeper/block_rewards_components.go#L10
• ...