This doesn't work at all because it relies on finding an scroller within
the <tree>, which we never have (nothing sets overflow to hidden/auto
anywhere where it'd work).
Remove this code which makes it a bit easier to reason about the tree
element.
Also, remove a redundant box (since the <treechildren> already flexes)
Differential Revision: https://phabricator.services.mozilla.com/D226129
