riscv: remove unused __ARCH_HAVE_MMU define

The __ARCH_HAVE_MMU define is (and was) used nowhere in the tree and
also doesn't appear to be used by any libc.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/unistd.h

@@ -11,7 +11,6 @@
  *   GNU General Public License for more details.
  */
 
-#define __ARCH_HAVE_MMU
 #define __ARCH_WANT_SYS_CLONE
 #include <uapi/asm/unistd.h>
 #include <uapi/asm/syscalls.h>