I found the answer myself:
There is a toolbars.cfg in \browser\defaults\settings. By uncommenting the relevant section the button can be removed.
Yeah this is the DEFAULT file, but attention, it's valid for all new profiles too. So a safer way would be to copy it into the current skin folder before customizing anything.
Tip: It's possible to manually create an own empty folder next to the other skin folders, name as wished (myskin123... whatever). Copy into it either all OR only customized files (cfg and/or images). Restart KM, choose that "new skin" in F2>GUI, and restart again.