[Cuttlefish] Remove last traces of themes and plasma themes

Summary:
As far as I can tell plasma theme support was only used for a checkbox to override
icons with plasma icons from the current plasma theme. Also icons with names in
the plasma but not icon theme were shown at the bottom of the grid. This was
removed with 727fd286. Themeexplorer is probably
the right place anyways to view plasma theme icons. Different icon themes were
never supported?

Test Plan: No difference

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27136