[Kicker/Kickoff] Use better "Remove from Favorites" icon

Summary:
Don't use a minus, because there is no plus next to it.
{F7127189}

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

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22861