csky: Remove unused core-y for dts

This line was used for embedding a DT into vmlinux.

Since commit c4c14c3b ("csky: remove builtin-dtb Kbuild"),
DT for csky is just a separate blob.

It is covered by the generic rule in the top Makefile:

  ifdef CONFIG_OF_EARLY_FLATTREE
  all: dtbs
  endif
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Signed-off-by: NGuo Ren <guoren@linux.alibaba.com>

arch/csky/Makefile

@@ -69,7 +69,6 @@ libs-y += arch/csky/lib/ \
 	$(shell $(CC) $(KBUILD_CFLAGS) $(KCFLAGS) -print-libgcc-file-name)
 
 boot := arch/csky/boot
-core-y += $(boot)/dts/
 
 all: zImage