[applets/systemtray] Clear item from shown/hidden list when disabling entry

Summary:
Else the entry is still in cfg_shownItems/cfg_hiddenItems and the config UI will display it as shown instead of disabled.

BUG: 419197

Reviewers: Plasma, kmaterka, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28311