[mle] simplify requesting shorter Child ID Request msg (#11719)

This commit inlines `RequestShorterChildIdRequest()`  into its only
call site within `HandleChildIdRequestTxDone()`.

src/core/thread/mle.cpp

@@ -1751,15 +1751,6 @@ exit:
     FreeMessageOnError(message, error);
 }
 
-void Mle::RequestShorterChildIdRequest(void)
-{
-    if (mAttachState == kAttachStateChildIdRequest)
-    {
-        mAddressRegistrationMode = kAppendMeshLocalOnly;
-        IgnoreError(SendChildIdRequest());
-    }
-}
-
 void Mle::HandleChildIdRequestTxDone(Message &aMessage)
 {
     if (aMessage.GetTxSuccess() && !IsRxOnWhenIdle())
@@ -1768,7 +1759,7 @@ void Mle::HandleChildIdRequestTxDone(Message &aMessage)
         Get<MeshForwarder>().SetRxOnWhenIdle(false);
     }
 
-    if (aMessage.IsLinkSecurityEnabled())
+    if (aMessage.IsLinkSecurityEnabled() && (mAttachState == kAttachStateChildIdRequest))
     {
         // If the Child ID Request requires fragmentation and therefore
         // link layer security, the frame transmission will be aborted.
@@ -1777,7 +1768,9 @@ void Mle::HandleChildIdRequestTxDone(Message &aMessage)
         // address in the Address Registration TLV).
 
         LogInfo("Requesting shorter `Child ID Request`");
-        RequestShorterChildIdRequest();
+
+        mAddressRegistrationMode = kAppendMeshLocalOnly;
+        IgnoreError(SendChildIdRequest());
     }
 }
 

src/core/thread/mle.hpp

@@ -641,15 +641,6 @@ public:
      */
     void HandleChildIdRequestTxDone(Message &aMessage);
 
-    /**
-     * Requests MLE layer to prepare and send a shorter version of Child ID Request message by only
-     * including the mesh-local IPv6 address in the Address Registration TLV.
-     *
-     * Should be called when a previous MLE Child ID Request message would require fragmentation at 6LoWPAN
-     * layer.
-     */
-    void RequestShorterChildIdRequest(void);
-
     /**
      * Schedules a Child Update Request.
      */