Don't accept hover events in the DragArea

Summary:
We don't actually process hoverEnter/move events, so there's no need to
have the overhead of having them sent to use.

We still get dragMove events regardless.

Setting it enabled broke FolderView nested in a grouping Plasmoid and
doesn't do anything useful.

Test Plan:
Tested folderview in a grouping plasmoid
Also tested dragging an item from dolphin over an existing folderview.
Dragging over folders still worked as before

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: hein, aacid, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D5573