Hide "Lock Widgets" button from panel edit mode toolbar

Summary:
In D24839 we hide "Lock Widgets" from the UI, so do the same thing here.

BUG: 412433

Test Plan: Open the panel edit mode.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: VDG, plasma-devel, Plasma

Tags: Plasma

Maniphest Tasks: T10190, T11094, T10402

Differential Revision: https://phabricator.kde.org/D24162