[kcmkwin] Remove redundant lambda
Summary:
We don't need a lambda because QDialog::accepted can be connected
directly to KCModule::save.
Test Plan: Compiles.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D28443