[Kicker/Kickoff] Use better "Remove from Favorites" icon
Don't use a minus, because there is no plus next to it.
Test Plan: Open Kickoff and open the context menu of a favorite application item.
Reviewers: VDG, ndavis, ngraham, hein
Reviewed By: VDG, ngraham, hein
Subscribers: plasma-devel, VDG
Differential Revision: https://phabricator.kde.org/D22861