Fix autologin session loading

Summary:
SDDM config stored the autologin value to be in the form

Session=Plasma

not

Session=Plasma.desktop

This patch strips the .desktop argument away in our list so that initial
selection works.

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18765