[netdata] remove mType in NetworkData class (#6977)

This commit simplifies the `NetworkData` class by removing the `mType`
Instead we add a new parameter in `SendServerDataNotification()` which
indicates whether to append the Network Data TLV to the message or
not (which was previously determined by `mType`).

tests/unit/test_network_data.cpp

@@ -68,7 +68,7 @@ void TestNetworkDataIterator(void)
     {
     public:
         TestNetworkData(ot::Instance *aInstance, const uint8_t *aTlvs, uint8_t aTlvsLength)
-            : NetworkData(*aInstance, kTypeLeader)
+            : NetworkData(*aInstance)
         {
             memcpy(mTlvs, aTlvs, aTlvsLength);
             mLength = aTlvsLength;