How to change shortcuts from older versions?

I had set “s” for split region in Ardour 4 and it worked for me in loder A5 versions as well. But now hiting s does no split anymore.
Now, when I try to set “s” for split region in the shortcut editor of A5.8 it tells me, that would be set for split already and I shall unset this before I can use that stroke for the presumaby new action.

How can I unset the legacy shortcut?

Instrucions here: