um: remove redundant generic-y

This commit removes redundant generic-y defines in
arch/um/include/asm/Kbuild.

It is redundant to define generic-y when arch-specific implementation
exists in arch/$(ARCH)/include/asm/*.h

Remove the following generic-y:

    hardirq.h
    io.h

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Richard Weinberger <richard@nod.at>