[GTK Subpage] Hide Breeze Dark for GTK3

Summary:
Given, that Breeze GTK grabs its colors from color scheme and Breeze
Dark doesn't do that creating by that confusion within the users, dark
theme should be hidden from the list of available GTK3 themes.

Keeping dark variant may be useful for other desktop environments where
there is no dynamic recoloring and there aren't any color scheme settings.

CCBUG: 413198

Test Plan: Check if the GTK3 combobox in Application Style > GNOME/GTK Application Style does not have the "Breeze-Dark" option.

Reviewers: cblack

Reviewed By: cblack

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29176