[applets/digital-clock] Remove the tooltip icon

Summary:
To increase consistency and reduce redundancy, widget tooltip icons are being removed
in D27847 and D27848, per discussion in T12778. This patch makes the Digital Clock
consistent what that initiative, since it has a custom tooltip.

Test Plan:

Reviewers: VDG, Plasma, niccolove

Reviewed By: niccolove

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T12778

Differential Revision: https://phabricator.kde.org/D27849