[sddm-theme] Remove blur on login screen when pressing a modifier

Summary:
Clone of b9f9581f99ef3f50c30b26d87a0258e0d2531f4d which was only applied
to the lock screen.

CCBUG: 402957

Test Plan: sddm-greeter --test-mode --theme

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17996