[mlr] remove unnecessary calls to UpdateMldReport (#7167)

This commit removes incorrect calls to UpdateMldReport.

- UpdateMldReport(MA, true) should be called when a MA is registered,
  thus in MulticastRoutingManager::Add.

- UpdateMldReport(MA, false) should be called when a MA is expired,
  thus in MulticastRoutingManager::Remove.

Note that removing an Multicast Forwarding Cache (MFC) does not mean
the MA is expired. The MFCs can be freely removed even when the MA is
still registered and being renewed.

src/posix/platform/multicast_routing.cpp

@@ -370,7 +370,6 @@ void MulticastRoutingManager::RemoveInboundMulticastForwardingCache(const Ip6::A
     {
         if (mfc.IsValid() && mfc.mIif == kMifIndexBackbone && mfc.mGroupAddr == aGroupAddr)
         {
-            UpdateMldReport(mfc.mGroupAddr, false);
             RemoveMulticastForwardingCache(mfc);
         }
     }
@@ -395,7 +394,6 @@ void MulticastRoutingManager::ExpireMulticastForwardingCache(void)
         {
             if (!UpdateMulticastRouteInfo(mfc))
             {
-                UpdateMldReport(mfc.mGroupAddr, false);
                 // The multicast route is expired
                 RemoveMulticastForwardingCache(mfc);
             }
@@ -561,7 +559,6 @@ void MulticastRoutingManager::SaveMulticastForwardingCache(const Ip6::Address &
     }
     else
     {
-        UpdateMldReport(oldest->mGroupAddr, false);
         RemoveMulticastForwardingCache(*oldest);
         oldest->Set(aSrcAddr, aGroupAddr, aIif, aOif);
     }