sddm-theme: Set needsFullUserModel=false

Summary:
Otherwise SDDM has to load all users even if they're not shown.
See https://github.com/sddm/sddm/pull/1017

Test Plan: Using this for several releases downstream to support >1500 users.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, davidedmundson, filipf, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24333