kconfig: remove duplicated CMD_DNS option

two CMD_DNS options were added by commit 60296a835cb17 ("commands: add more
command entries in Kconfig")

Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/common/Kconfig b/common/Kconfig
index 17930a4..5d7e48a 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -300,11 +300,6 @@
 	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