sunxi: remove SUNXI_UBOOT from DEVICE_VARS

All SUNXI_UBOOT usages were removed in e018c4d7ab ("uboot-sunxi: clean up,
switch to u-boot.mk"). Now drop the variable completely.

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

target/linux/sunxi/image/Makefile

@@ -32,7 +32,7 @@ endef
 # why \x00\x00\x00\x00 for zImage-initramfs
 define Device/Default
   PROFILES := Default
-  DEVICE_VARS := SUNXI_DTS SUNXI_DTS_DIR SUNXI_UBOOT
+  DEVICE_VARS := SUNXI_DTS SUNXI_DTS_DIR
   KERNEL_NAME := zImage
   KERNEL := kernel-bin | uImage none
   IMAGES := sdcard.img.gz