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>

Makefile

@@ -399,11 +399,6 @@ ifeq ($(ARCH),sparc64)
        SRCARCH := sparc
 endif
 
-# Additional ARCH settings for sh
-ifeq ($(ARCH),sh64)
-       SRCARCH := sh
-endif
-
 export cross_compiling :=
 ifneq ($(SRCARCH),$(SUBARCH))
 cross_compiling := 1