[PlasmaComponents]Clean up ListItem code

Summary:
Removed two unused lines.
Removed old comments.
Minor style changes to comments.

Test Plan: Everything must stay unchanged visually and function wise for ListItems.

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27296