[topology] remove unused (non const) GetExtAddress() method (#4249)

src/core/thread/topology.hpp

@@ -163,14 +163,6 @@ public:
      */
     void ClearExtAddress(void) { memset(&mMacAddr, 0, sizeof(mMacAddr)); }
 
-    /**
-     * This method returns the Extended Address.
-     *
-     * @returns A reference to the Extended Address.
-     *
-     */
-    Mac::ExtAddress &GetExtAddress(void) { return mMacAddr; }
-
     /**
      * This method returns the Extended Address.
      *