The function is rather useless in tree-view if you want to groom the view to have a better focus on a few items that you want to compare, as you cannot remove multiple lines quickly. Either it should allow you to select multiple lines (doing that with shift yields another bug) or it should advance the selection so you can remove multiple lines quickly by hitting the remove key multiple times. I'd rather prefer the first solution - but maybe you can come up with something much better.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.