cpu: Remove CPU_COMMON

This macro is now always empty, so remove it.  This leaves the
entire contents of CPUArchState under the control of the guest
architecture.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/exec/cpu-defs.h

@@ -231,8 +231,6 @@ typedef struct CPUTLB { } CPUTLB;
 
 #endif  /* !CONFIG_USER_ONLY && CONFIG_TCG */
 
-#define CPU_COMMON  /* Nothing */
-
 /*
  * This structure must be placed in ArchCPU immedately
  * before CPUArchState, as a field named "neg".