Don't set X-Plasma-MainScript for wallpaper plugins

Summary:
The file path of the main script is hard-coded to ui/main.qml, so setting
X-Plasma-MainScript is pointless.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25201