In the admin Control panel > Content > bbcode button manager.
Click on Large, medium, small extra small toolbars and drag the icon form the available icons at the top onto or off of the representation of the toolbar.
You have various different sizes of toolbar for different display widths
Pity. In that case you'll need to dig into the js code
Here is an old tutorial (xenForo 1) that might give you some clues how to do it with xenForo 2 if you are brave enough!
Bear in mind the next xenForo upgrade is going to have a new editor, so if you were to spend time/money on custom development in this regard then it may all change soon.
Bear in mind the next xenForo upgrade is going to have a new editor, so if you were to spend time/money on custom development in this regard then it may all change soon.