Use the default Plasma wallpaper on the login screen

Summary:
As discussed and agreed to by VDG (T7914), change the SDDM theme background from the blue color to the current Plasma wallpaper.

BUG: 381407
FIXED-IN: 5.13.0

Test Plan:
- Build and deploy plasma-workspace with this patch
- Log out

{F5753281}

Reviewers: #plasma, #vdg, broulik, abetts, ngraham, mart

Reviewed By: #plasma, #vdg, abetts, ngraham, mart

Subscribers: mart, davidedmundson, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11309