nios2: remove unneeded arch/nios2/include/(generated/)asm/signal.h

nios2: remove unneeded arch/nios2/include/(generated/)asm/signal.h

Currently, NIOS2 has three signal.h files under arch/nios2/include:

[1] arch/nios2/include/asm/signal.h
[2] arch/nios2/include/uapi/asm/signal.h
[3] arch/nios2/include/generated/asm/signal.h

[3] is build-time generated by scripts/Makefile.asm-generic.
However, -I$(srctree)/arch/$(hdr-arch)/include search path is listed
before -I$(objtree)/arch/$(hdr-arch)/include/generated in LINUXINCLUDE.
Therefore [1] is always included instead of [3].  Remove [3] which
is never included.

If we look at [1], it just includes [2].  So, [1] can be removed
as well.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>