darwin-user/main.c: Drop unused cpu_single_env definition

Drop the cpu_single_env definition as it is unused.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

darwin-user/main.c

@@ -729,8 +729,6 @@ static void usage(void)
 
 /* XXX: currently only used for async signals (see signal.c) */
 CPUState *global_env;
-/* used only if single thread */
-CPUState *cpu_single_env = NULL;
 
 /* used to free thread contexts */
 TaskState *first_task_state;