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

arch/s390/tools/Makefile

@@ -14,8 +14,7 @@ kapi:	$(kapi-hdrs-y)
 hostprogs-y		    += gen_facilities
 hostprogs-y		    += gen_opcode_table
 
-HOSTCFLAGS_gen_facilities.o += -Wall $(LINUXINCLUDE)
-HOSTCFLAGS_gen_opcode_table.o += -Wall
+HOSTCFLAGS_gen_facilities.o += $(LINUXINCLUDE)
 
 # Ensure output directory exists
 _dummy := $(shell [ -d '$(kapi)' ] || mkdir -p '$(kapi)')