csky: remove meaningless header-y

Since commit fcc8487d

 ("uapi: export all headers under uapi
directories"), header-y is meaningless because headers under uapi
are all exported.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Guo Ren <guoren@kernel.org>