Remove scaling of iconsize unit for isMobile

Summary:
On Android icons using the iconSizes units look too big for the intended size, this change has been tested on different screen sizes with devices running Android and also on a Nexus 5x running Plamo, and the result is goo. I'm attaching the resulting screenshots and screencast.

on a 3.5" screen. Also, the big icons would be an issue on such small screens, since they would take all the space and everything would look too bulky.

Reviewers: VDG, mart, ngraham

Reviewed By: VDG, mart, ngraham

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D22572