zetacore [observer]: improve logic in the staking hooks

[kingpinXD]

The logic present in the staking hooks , is responsible for removing observers from the observer set , if their stake falls below the minimum required amount
https://github.com/zeta-chain/node/blob/develop/x/observer/keeper/hooks.go

This logic can be refactored to make it more readable by abstracting out some of the common logic.
We should also add more unit tests to test these functions, including all possible edge cases.