kconfig: qconf: remove unused ConfigItem::okRename()

Commit 76538660 ("Port xconfig to Qt5 - Remove custom
ListView classes.") removed the original implementation, where
ConfigItem::okRename() overrode Q3ListViewItem::okRename().

Commit 59e56440

 ("Port xconfig to Qt5 - Put back some of the
old implementation.") restored the empty stub, but it seems
useless.

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