csky: Remove unused $(dtb-y) from boot/Makefile

arch/csky/boot/Makefile does not build DTB,
arch/csky/boot/dts/Makefile does.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Guo Ren <guoren@kernel.org>

arch/csky/boot/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 targets := Image zImage uImage
-targets += $(dtb-y)
 
 $(obj)/Image: vmlinux FORCE
 	$(call if_changed,objcopy)