remove binding loop in KickoffItem

Summary:
For non-wrapped text, implicitHeight will be the height of the
paintedText. The key difference is it won't re-evaluate except on text
changing which removes the binding loop.

BUG: 402252

Test Plan:
Opened kickoff
Looked identical
No more warnings

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17642