scripts/Makefile.extrawarn: Do not disable clang's -Wformat-zero-length

There are no instances of this warning in the tree across several
difference architectures and configurations. This was added by
commit 26ea6bb ("kbuild, LLVMLinux: Supress warnings unless W=1-3")
back in 2014, where it might have been necessary, but there are no
instances of it now so stop disabling it to increase warning coverage
for clang.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.extrawarn

@@ -49,7 +49,6 @@ ifdef CONFIG_CC_IS_CLANG
 KBUILD_CFLAGS += -Wno-initializer-overrides
 KBUILD_CFLAGS += -Wno-format
 KBUILD_CFLAGS += -Wno-sign-compare
-KBUILD_CFLAGS += -Wno-format-zero-length
 KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
 KBUILD_CFLAGS += -Wno-tautological-constant-out-of-range-compare
 KBUILD_CFLAGS += $(call cc-disable-warning, unaligned-access)