Don't set ItemHasContents in DropArea

Summary:
ItemHasContents means we get included in generating the QSG node
treeview when we render.

This item doesn't have any contents. It's like a MouseArea.

::setEnabled() doesn't need updating as it was already missing.

Reviewers: Plasma, aacid

Reviewed By: aacid

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5574