kbuild: add -Wno-unused-but-set-variable flag unconditionally

We have raised the compiler requirement from time to time.
With commit cafa0010 ("Raise the minimum required gcc version
to 4.6"), the minimum for GCC is 4.6 now.

This flag was added by GCC 4.6, and it is recognized by ICC as well.

It is true that Clang does not support this flag but this commit is
just touching the else part of the "ifeq ($(cc-name),clang)" check.
Hence, Clang build is not affected.

Let's rip off the cc-disable-warning switch, and see if somebody
complains about it.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -723,7 +723,7 @@ else
 
 # These warnings generated too much noise in a regular build.
 # Use make W=1 to enable them (see scripts/Makefile.extrawarn)
-KBUILD_CFLAGS += $(call cc-disable-warning, unused-but-set-variable)
+KBUILD_CFLAGS += -Wno-unused-but-set-variable
 endif
 
 KBUILD_CFLAGS += $(call cc-disable-warning, unused-const-variable)