[Notifications] Unset Heading default height

Plasma's Label has a default height based on the contentHeight of the text.
However, this interferes with QtQuick Layout's logic and might cause an infinite loop.

This can easily be triggered by sending a notification with a multi-line heading/summary.

CHANGELOG: Fixed a freeze caused by certain notifications

BUG: 381154
FIXED-IN: 5.12.0
(cherry picked from commit a3c0565ace32ce91cc0c5ef2528b19fe9d8635d8)