mt7620: disable image generation for Nexx WT3020 (4MB)

Image generation is currently failing on builbots due to the following
error:

WARNING: Image file openwrt-19.07-snapshot-r10495-db5164d3d0-ramips-mt7620-wt3020-4M-squashfs-factory.bin is too big

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/ramips/image/mt7620.mk

@@ -730,7 +730,7 @@ define Device/wt3020-4M
 	poray-header -B WT3020 -F 4M
   DEVICE_TITLE := Nexx WT3020 (4MB)
 endef
-TARGET_DEVICES += wt3020-4M
+#TARGET_DEVICES += wt3020-4M
 
 define Device/wt3020-8M
   DTS := WT3020-8M