kcm/keys: don't allow collapsing single shortcut ation list items

The purpose of collapsibility is to have only one item at a time
expanded in multi-item lists. But when there's only one item, this
doesn't make sense, and the button doesn't even do anythign when
clicked. So let's hide it.

BUG: 462141
FIXED-IN: 5.26.5


(cherry picked from commit f05593c2)