package/gtk2-themes: remove

The category just contains gtk2-theme-hicolor... which is a duplicate of
hicolor-icon-theme, d'oh!
Remove it and select the only one via legacy.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>