ramips: drop LinkIt Smart 7688 Duo userspace support

The  LinkIt Smart 7688/LinkIt Smart 7688 Duo are identical beside the
extra ATmega32U4 - accessible via UART - on the the Duo.

Since all relevant hardware is identical, drop the Duo special handling
in userspace.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/ramips/base-files/lib/ramips.sh

@@ -278,13 +278,7 @@ ramips_board_detect() {
 		name="m4-8M"
 		;;
 	*"MediaTek LinkIt Smart 7688")
-		linkit="$(dd bs=1 skip=1024 count=12 if=/dev/mtd2 2> /dev/null)"
-		if [ "${linkit}" = "LINKITS7688D" ]; then
-			name="linkits7688d"
-			RAMIPS_MODEL="${machine} DUO"
-		else
-			name="linkits7688"
-		fi
+		name="linkits7688"
 		;;
 	*"Memory 2 Move")
 		name="m2m"