ipq40xx: image: remove unused DTB_SIZE variable

It is not included in DEVICE_VARS anyways.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ipq40xx/image/generic.mk

@@ -217,7 +217,6 @@ define Device/asus_rt-ac42u
 	SOC := qcom-ipq4019
 	BLOCKSIZE := 128k
 	PAGESIZE := 2048
-	DTB_SIZE := 65536
 	IMAGE_SIZE := 20439364
 	FILESYSTEMS := squashfs
 #	RT-AC82U is nowhere to be found online
@@ -237,7 +236,6 @@ define Device/asus_rt-ac58u
 	SOC := qcom-ipq4018
 	BLOCKSIZE := 128k
 	PAGESIZE := 2048
-	DTB_SIZE := 65536
 	IMAGE_SIZE := 20439364
 	FILESYSTEMS := squashfs
 #	Someone - in their infinite wisdom - decided to put the firmware