target-arm/exec.h: Remove unused #define of M0

Remove a preprocessor #define which is never used.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/exec.h

@@ -21,8 +21,6 @@
 
 register struct CPUARMState *env asm(AREG0);
 
-#define M0   env->iwmmxt.val
-
 #include "cpu.h"
 #include "exec-all.h"