[evm] Refactor out dead code from the evm.Params

[Unique-Divine]

Context

Current evm.Params structure (commit: 37913c4):

// From evm/params.go

func DefaultParams() Params {
	return Params{
		EvmDenom:            DefaultEVMDenom,
		ExtraEIPs:           []int64{},
		AllowUnprotectedTxs: false,
		EVMChannels:         []string{},
		CreateFuntokenFee:   math.NewIntWithDecimal(10_000, 6), // 10_000 NIBI
	}
}

• EvmDenom should be a consant since we're highly unlikely to ever change to
  anything besides NIBI, and having it flexible exposes the network to a governance
  attack that messes up the economics of Nibiru's EVM by switching the denom to a
  different bank coin.
• AllowUnprotectedTxs is always false. We should remove it
• EVMChannels is unused, as it was intended for extending the module with
  IBC functionality. We should document why it exists.