mediatek: remove ext4 images from mt7623 build

A full read-write rootfs cannot work inside uImage.FIT as the hash
will obviously change once writing to it. Disable generating ext4
rootfs images.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/mt7623/target.mk

@@ -8,7 +8,7 @@ BOARDNAME:=MT7623
 CPU_TYPE:=cortex-a7
 CPU_SUBTYPE:=neon-vfpv4
 KERNELNAME:=Image dtbs zImage
-FEATURES+=ext4 usb
+FEATURES+=usb
 KERNEL_PATCHVER:=5.10
 DEFAULT_PACKAGES+=blockdev uboot-envtools