[data-poll-sender] change StartPolling() to return void (#4941)

src/core/mac/data_poll_sender.cpp

@@ -69,18 +69,17 @@ const Neighbor &DataPollSender::GetParent(void) const
     return parentCandidate.IsStateValid() ? parentCandidate : Get<Mle::MleRouter>().GetParent();
 }
 
-otError DataPollSender::StartPolling(void)
+void DataPollSender::StartPolling(void)
 {
-    otError error = OT_ERROR_NONE;
+    VerifyOrExit(!mEnabled, OT_NOOP);
 
-    VerifyOrExit(!mEnabled, error = OT_ERROR_ALREADY);
-    VerifyOrExit(!Get<Mle::MleRouter>().IsRxOnWhenIdle(), error = OT_ERROR_INVALID_STATE);
+    OT_ASSERT(!Get<Mle::MleRouter>().IsRxOnWhenIdle());
 
     mEnabled = true;
     ScheduleNextPoll(kRecalculatePollPeriod);
 
 exit:
-    return error;
+    return;
 }
 
 void DataPollSender::StopPolling(void)

src/core/mac/data_poll_sender.hpp

@@ -79,12 +79,8 @@ public:
     /**
      * This method instructs the data poll sender to start sending periodic data polls.
      *
-     * @retval OT_ERROR_NONE            Successfully started sending periodic data polls.
-     * @retval OT_ERROR_ALREADY         Periodic data poll transmission is already started/enabled.
-     * @retval OT_ERROR_INVALID_STATE   Device is not in rx-off-when-idle mode.
-     *
      */
-    otError StartPolling(void);
+    void StartPolling(void);
 
     /**
      * This method instructs the data poll sender to stop sending periodic data polls.

src/core/thread/mesh_forwarder.cpp

@@ -358,7 +358,7 @@ void MeshForwarder::SetRxOnWhenIdle(bool aRxOnWhenIdle)
     }
     else
     {
-        IgnoreError(mDataPollSender.StartPolling());
+        mDataPollSender.StartPolling();
         Get<Utils::SupervisionListener>().Start();
     }
 }