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: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>