disas/microblaze: Remove unused REG_SP macro

disas/microblaze: Remove unused REG_SP macro

This causes a build error with debian sid, riscv64 host:

disas/microblaze.c:179: error: "REG_SP" redefined [-Werror]
 #define REG_SP  1 /* stack pointer */

In file included from /usr/include/signal.h:306,
                 from include/qemu/osdep.h:101,
                 from disas/microblaze.c:36:
/usr/include/riscv64-linux-gnu/sys/ucontext.h:36: note: this is the location of the previous definition
 # define REG_SP 2

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>