Remove unused FocusTrap and ScrollBarHelper utilities (#42595)

Remove unused FocusTrap and ScrollBarHelper utilities (#42595)

Both became dead code with the Modal->Dialog / Offcanvas->Drawer rewrite:
the native <dialog> element provides the focus trap and top-layer
inerting, and the scroll lock is now :root.dialog-open in CSS. Nothing in
js/src imports either utility and neither is part of the public bundle
export — only their own unit specs referenced them.

Removes the source modules, their committed dist builds, and their specs.