Fix screen reader issues in hierarchy tree

[siiriylonen]

A couple of noted accessibility issues when using JAWS reader specifically:

On the first element (when entering the tree) it reads:
“Use JawsKey+Alt+M to move to controlled element”
aria-controls seemed to be the cause of this.
Could it be removed?

Also at the end points of hierarchy tree, it unnecessarily reads the disabled button.
Could the whole button be removed?

[EreMaijala]

@siiriylonen The disabled button cannot be removed without it affecting the layout. Perhaps a better solution would be to hide it from screen readers?

[demiankatz]

I'm marking this as "request changes" until @EreMaijala's comment is addressed, just to be sure it doesn't get merged prematurely by accident. :-) Thank you, @siiriylonen!