Fix bug where a received message would remove completions for users
`Array.prototype.splice` will return the array of removed items, not a new array. The array operated on is actually modified in-place.
This was causing a few weird things to happen: https://github.com/vector-im/riot-web/issues/4511 and https://github.com/vector-im/riot-web/issues/4533. This should fix both of them but it is concerning that doing the tab completion is required to reproduce. Let's just see how this goes before closing the issues.
Thanks @turt2live for reproducing both bugs, giving enough information for a fix :)