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 26ea6bb1

 ("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>