kbuild: Remove -Wno-format-invalid-specifier from clang block

Turning on -Wformat does not reveal any instances of this warning across
several different builds so remove this line to keep the number of
disabled warnings as slim as possible.

This has been disabled since commit 61163efa ("kbuild: LLVMLinux:
Add Kbuild support for building kernel with Clang"), which does not
explain exactly why it was turned off but since it was so long ago in
terms of both the kernel and LLVM so it is possible that some bug got
fixed along the way.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -782,7 +782,6 @@ KBUILD_CFLAGS += $(stackp-flags-y)
 
 ifdef CONFIG_CC_IS_CLANG
 KBUILD_CPPFLAGS += -Qunused-arguments
-KBUILD_CFLAGS += -Wno-format-invalid-specifier
 KBUILD_CFLAGS += -Wno-gnu
 # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
 # source of a reference will be _MergedGlobals and not on of the whitelisted names.