target-arm: Remove unused ARMCPUState sr substruct

Remove the 'struct sr' from ARMCPUState -- it isn't actually used and is
a hangover from the original separate system register implementation used
by the SuSE linux-user-mode-only AArch64 target.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>

target-arm/cpu.h

@@ -217,11 +217,6 @@ typedef struct CPUARMState {
         uint32_t c15_power_control; /* power control */
     } cp15;
 
-    /* System registers (AArch64) */
-    struct {
-        uint64_t tpidr_el0;
-    } sr;
-
     struct {
         uint32_t other_sp;
         uint32_t vecbase;