kbuild: remove EXTRA_*FLAGS support

Commit f77bf014 ("kbuild: introduce ccflags-y, asflags-y and
ldflags-y") deprecated these in 2007. The migration should have been
completed by now.

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