kconfig: qconf: remove wrong ConfigList::firstChild()

This function returns the first child object, but the returned pointer
is not compatible with (ConfigItem *).

Commit cc1c08edcc ("kconfig: qconf: don't show goback button on
splitMode") uncovered this issue because using the pointer from this
function would make qconf crash. (https://lkml.org/lkml/2020/7/18/411)

This function does not work. Remove.

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

scripts/kconfig/qconf.h

@@ -92,10 +92,6 @@ public:
 	{
 		return this;
 	}
-	ConfigItem* firstChild() const
-	{
-		return (ConfigItem *)children().first();
-	}
 	void addColumn(colIdx idx)
 	{
 		showColumn(idx);