Remove unneeded options in GTK KCM

Summary:
Since font, icons, cursor theme, toolbar style, icons in menus and buttons settings are now in sync with corresponding plasma settings, they shouldn't be in GTK KCM anymore.

Depends on D24743

BUG: 406740

BUG: 401008

BUG: 401123

FIXED-IN: 5.18.0

Test Plan:
Check if nothing breaks in GTK KCM functionality, that is left.

Reviewers: VDG, Plasma, ngraham, nicolasfella

Reviewed By: VDG, ngraham, nicolasfella

Subscribers: apol, ngraham, broulik, filipf, GB_2, plasma-devel

Tags: Plasma, VDG

Maniphest Tasks: T10611

Differential Revision: https://phabricator.kde.org/D24744