Don't define STOLEN_X86_REGS in Cmm.h

We weren't defining it in the other places that MachRegs.h gets
imported, which seems a little suspicious. And if it's not defined
then it defaults to 4 anyway, so this define doesn't seem necessary.

includes/Cmm.h

@@ -346,7 +346,6 @@
  * Need MachRegs, because some of the RTS code is conditionally
  * compiled based on REG_R1, REG_R2, etc.
  */
-#define STOLEN_X86_REGS 4
 #include "stg/MachRegs.h"
 
 #include "rts/storage/Liveness.h"