py/objgenerator: Remove unreachable code for STOP_ITERATION case.

This commit essentially reverts aa9dbb1b03
where this if-condition was added.  It seems that even when that commit
was made the code was never reached by any tests, nor reachable by
analysis (see below).  The same is true with the code as it currently
stands: no test triggers this if-condition, nor any uasyncio examples.
Analysing the flow of the program also shows that it's not reachable:

==START==
-> to trigger this if condition mp_execute_bytecode() must return
   MP_VM_RETURN_YIELD with *sp==MP_OBJ_STOP_ITERATION

   -> mp_execute_bytecode() can only return MP_VM_RETURN_YIELD from the
      MP_BC_YIELD_VALUE bytecode, which can happen in 2 ways:

      -> 1) from a "yield <x>" in bytecode, but <x> must always be a proper
         object, never MP_OBJ_STOP_ITERATION; ==END1==

      -> 2) via yield from, via mp_resume() which must return
         MP_VM_RETURN_YIELD with ret_value==MP_OBJ_STOP_ITERATION, which
         can happen in 3 ways:

         -> 1) it delegates to mp_obj_gen_resume(); go back to ==START==

         -> 2) it returns MP_VM_RETURN_YIELD directly but with a guard that
            ret_val!=MP_OBJ_STOP_ITERATION; ==END2==

         -> 3) it returns MP_VM_RETURN_YIELD with ret_val set from
            mp_call_method_n_kw(), but mp_call_method_n_kw() must return a
            proper object, never MP_OBJ_STOP_ITERATION; ==END3==

The above shows there is no way to trigger the if-condition and it can be
removed.

py/objgenerator.c

@@ -125,9 +125,6 @@ mp_vm_return_kind_t mp_obj_gen_resume(mp_obj_t self_in, mp_obj_t send_value, mp_
 
         case MP_VM_RETURN_YIELD:
             *ret_val = *self->code_state.sp;
-            if (*ret_val == MP_OBJ_STOP_ITERATION) {
-                self->code_state.ip = 0;
-            }
             break;
 
         case MP_VM_RETURN_EXCEPTION: {