arch/sim/src/sim/up_exit.c: Appease a clang-check warning

clang-check complained like the following.
While this is not a real bug, it's easy to appease.

sim/up_exit.c:68:21: warning: Value stored to 'tcb' during its initialization is never read
  FAR struct tcb_s *tcb = this_task();
                    ^~~   ~~~~~~~~~~~
1 warning generated.

arch/sim/src/sim/up_exit.c

@@ -65,9 +65,9 @@
 
 void up_exit(int status)
 {
-  FAR struct tcb_s *tcb = this_task();
+  FAR struct tcb_s *tcb;
 
-  sinfo("TCB=%p exiting\n", tcb);
+  sinfo("TCB=%p exiting\n", this_task());
 
   /* Destroy the task at the head of the ready to run list. */