ramips: fix SUPPORTED_DEVICES for Mercury MAC1200R v2

Currently SUPPORTED_DEVICES only contains the old device string. Fix it by
removing the first assignment.

Fixes: c2334ad60dc8 ("ramips/mt76x8: Synchronize Makefiles with DTS compatible")
Signed-off-by: Sungbo Eo <[email protected]>

target/linux/ramips/image/mt76x8.mk

@@ -161,7 +161,6 @@ define Device/mercury_mac1200r-v2
   DEVICE_VENDOR := Mercury
   DEVICE_MODEL := MAC1200R
   DEVICE_VARIANT := v2.0
-  SUPPORTED_DEVICES := mac1200rv2
   DEVICE_PACKAGES := kmod-mt76x2
   SUPPORTED_DEVICES += mac1200rv2
 endef