kconfig: gconf: remove unneeded variables in on_treeview*_button_press_event()

Not all position parameters are used here. Passing NULL to
gtk_tree_view_get_cursor() or gtk_tree_view_get_path_at_pos() is
allowed. [1] [2]

[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/2.24.33/gtk/gtktreeview.c#L12638
[1]: https://gitlab.gnome.org/GNOME/gtk/-/blob/2.24.33/gtk/gtktreeview.c#L12795



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