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

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

Commit cc1c08ed ("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.cc

@@ -434,7 +434,7 @@ void ConfigList::updateList(ConfigItem* item)
 	}
 	if ((mode == singleMode || (mode == symbolMode && !(rootEntry->flags & MENU_ROOT))) &&
 	    rootEntry->sym && rootEntry->prompt) {
-		item = last ? last->nextSibling() : firstChild();
+		item = last ? last->nextSibling() : nullptr;
 		if (!item)
 			item = new ConfigItem(this, last, rootEntry, true);
 		else

scripts/kconfig/qconf.h

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