kconfig: gconf: remove unneeded variable in on_split_clicked()

The height of the window is not used here. Passing NULL to
gtk_window_get_default_size() is allowed. [1]

[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/2.24.33/gtk/gtkwindow.c#L3974



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>