[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