gnome-system-monitor: Remove pkg-config check for specific icon theme

Also rrecommend adwaita-icon-theme instead of gnome-icon-theme
(as latter is removed from oe-core).

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>