when deleting an entry, also delete the widgets

previously what would happen is that KCMHotkeys::currentChanged would
run, find the new index (-1,-1) now invalid and show the global settings
instead. this however left the simple_action's underlying widgets still
sitting around referring to the previous index AND holding a dangling
trigger point that would eventually crash when the simple_action would
be poked by anything

instead force unset the internal state of the simple_action when showing
the global settings.

FIXED-IN: 5.23.5
BUG: 443656


(cherry picked from commit 97f9339f)