[mobile/wifi] Use default height for ConnectionItemDelegate

Summary: Using the default height of SwipeListItem gives a more sensible and consistent with other lists look

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, jgrulich, mkacej, ngraham

Reviewed By: VDG, jgrulich, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22180