Reset tooltip delegate label height to defaults
It seems Qt 5.12 changed the behavior of contentHeight to not include any font padding.
This is also supposedly fixed in Qt 5.12.1 but this change still makes sense since
assigning contentHeight effectively resets the height anyway so we might as well do it properly.
BUG: 401842
FIXED-IN: 5.15.3
Differential Revision: https://phabricator.kde.org/D18640