s390: drop unneeded -Wall addition from tools Makefile

The top level Makefile adds -Wall globally for all host tools:

  KBUILD_HOSTCFLAGS   := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \

I see two "-Wall" added for compiling these tools.

Of course, it is allowed to pass the same option multiple times, but
we do not need to do so.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>