kbuild: remove ARCH=sh64 support from top Makefile

Commit 37744fee ("sh: remove sh5 support") removed the SUPERH64
support entirely.

Remove the left-over code from the top Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>