binfmt_flat: Drop vestiges of coredump support

There is the briefest start of coredump support in binfmt_flat. It
is actually a pain to maintain as binfmt_flat is not built on most
architectures so it is easy to overlook.

Since the support does not do anything remove it.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/87mtgh17li.fsf_-_@email.froward.int.ebiederm.org