Remove _for_theme() variant of st_texture_cache_load_icon_name()

Now that we're using St.Icon in the Javascript, there is no reason
to have separate st_texture_cache_load_icon_name() and
st_texture_cache_load_icon_name_for_theme(), instead just add
the StThemeNode argument to st_texture_cache_load_icon_name().

https://bugzilla.gnome.org/show_bug.cgi?id=633866