startplasma: Respect KDEWM again

Summary: It got lost during the porting to native executables

Test Plan:
Set KDEWM=awesome, successfully started a new Plasma
session with it, unset it again and kwin is started in the next
session.

Reviewers: Plasma, apol, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23910