Remove some spacing on lockscreen that was causing elements to get cut off on small displays

Summary:
This patch removes some spacing which was making the 'switch users' button get slightly cut off on small displays (1366x768). Quite a few old laptops have screens of this size, so it is good to take into account that display size as well, as long as it doesn't negatively affect the appearance for large displays.

BUG: 410801
FIXED-IN: 5.17.0

Test Plan:
Lock screen when connected to a low-res display and check that the 'switch users' button isn't cut off.

Before this patch:
Lockscreen on small display (1366x768):

Lockscreen on big display (1920x1080):

After this patch:
Lockscreen on small display (1366x768):

Lockscreen on big display (1920x1080):

Reviewers: Plasma, VDG, ngraham, filipf

Reviewed By: VDG, ngraham, filipf

Subscribers: filipf, ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23786