[Login and lock screens] Do not use light font styles for the sddm clock

Summary:
This patch removes the line of code that required that the lock screen use a light font style.
This patch is also closely related to the fixes in D17905, D17906 and D17907.

Test Plan:
Worst case scenario (legibility extremely poor):

Before and after the patch:

Reviewers: ngraham, VDG, Plasma

Reviewed By: ngraham, VDG

Subscribers: abetts, davidedmundson, filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17916