ENR update call does not remove fields that are no longer applicable

https://github.com/status-im/nim-eth/blob/f133d7ab8d77d60025a0b028d76014baf5a4d9fa/eth/p2p/discoveryv5/enr.nim#L279 is flawed in the sense that old `fieldPairs` that are no longer in the new `fieldPairs` don't get removed. The update basically only works in 1 direction.