kconfig: remove duplicated CMD_DNS option

two CMD_DNS options were added by commit 60296a83 ("commands: add more
command entries in Kconfig")

Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>

common/Kconfig

@@ -300,11 +300,6 @@ config CMD_DNS
 	help
 	  Lookup the IP of a hostname
 
-config CMD_DNS
-	bool "dns"
-	help
-	  Lookup the IP of a hostname
-
 config CMD_LINK_LOCAL
 	bool "linklocal"
 	help