kconfig: remove a spurious self-assignment

The self assignment was probably introduced by an automated code
refactoring in
commit 694c49a7c0 ("kconfig: drop localization support").

The issue was identified by a self-assign warning when running
make menuconfig with clang.

Fixes: 694c49a7c0 ("kconfig: drop localization support")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/mconf.c

@@ -490,7 +490,6 @@ static void build_conf(struct menu *menu)
 			switch (prop->type) {
 			case P_MENU:
 				child_count++;
-				prompt = prompt;
 				if (single_menu_mode) {
 					item_make("%s%*c%s",
 						  menu->data ? "-->" : "++>",