Last icon on the button bar cannot be deleted

[RADiCAL]

If the button bar has only 1 icon, then you cannot delete it by right clicking on the icon & select Delete (the Delete is greyed-out).

Workaround
right click the empty space on the button bar -> select Change... & delete the icon from within the button bar window.

[ghisler(Author)]

This is by design, so you cannot destroy the button bar completely (and then no longer add any buttons because it's gone).