csky: remove redundant generic-y

Since commit 7cbbbb8b ("kbuild: warn redundant generic-y"),
redundant generic-y is reported. I missed to delete this one.

scripts/Makefile.asm-generic:25: redundant generic-y found in arch/csky/include/asm/Kbuild: ftrace.h

In this case, csky-specific implementation exists in
arch/csky/include/asm/ftrace.h
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Guo Ren <ren_guo@c-sky.com>

arch/csky/include/asm/Kbuild

@@ -12,7 +12,6 @@ generic-y += dma-mapping.h
 generic-y += emergency-restart.h
 generic-y += exec.h
 generic-y += fb.h
-generic-y += ftrace.h
 generic-y += futex.h
 generic-y += gpio.h
 generic-y += hardirq.h