[Containment Actions] Offer "Remove this panel" option only when panel is being edited

We only show the "Remove this applet" option when the panel controller is opened
to prevent the user from accidentally messing up his setup.
However, the "Remove this panel" option, albeit in a submenu, was always there and
is even more destructive.

BUG: 360300

Differential Revision: https://phabricator.kde.org/D1869