Revert "kconfig: qconf: don't show goback button on splitMode"

This reverts commit cc1c08ed.

Maxim Levitsky reports 'make xconfig' crashes since that commit
(https://lkml.org/lkml/2020/7/18/411

)

Or, the following is simple test code that makes it crash:

    menu "Menu"

    config FOO
            bool "foo"
            default y

    menuconfig BAR
            bool "bar"
            depends on FOO

    endmenu

Select the Split View mode, and double-click "bar" in the right
window, then you will see Segmentation fault.

When 'last' is not set for symbolMode, the following code in
ConfigList::updateList() calls firstChild().

  item = last ? last->nextSibling() : firstChild();

However, the pointer returned by ConfigList::firstChild() does not
seem to be compatible with (ConfigItem *), which seems another bug.

I'd rather want to reconsider whether hiding the goback icon is the
right thing to do.

In the following test code, the Split View shows "Menu2" and "Menu3"
in the right window. You can descend into "Menu3", but there is no way
to ascend back to "Menu2" from "Menu3".

    menu "Menu1"

    config FOO
            bool "foo"
            default y

    menu "Menu2"
            depends on FOO

    menu "Menu3"

    config BAZ
            bool "baz"

    endmenu

    endmenu

    endmenu

It is true that the goback button is currently not functional due to
yet another bug, but hiding the problem is not the right way to go.

Anyway, Segmentation fault is fatal. Revert the offending commit for
now, and we should find the right solution.

Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>