[BUGFIX] Emit layerWillBeRemoved like layersWillBeRemoved (#3194)

The signal layerWillBeremoved is only emitted when the layer is owned by QgsMapLayerRegistry.

To fix it just move the emitted layerWilBeRemoved out of the scope of layers owned by QgsMapLayerRegistry.

src/core/qgsmaplayerregistry.cpp

@@ -144,10 +144,10 @@ void QgsMapLayerRegistry::removeMapLayers( const QList<QgsMapLayer*>& layers )
       continue;
 
     QString myId( lyr->id() );
+    emit layerWillBeRemoved( myId );
+    emit layerWillBeRemoved( lyr );
     if ( mOwnedLayers.contains( lyr ) )
     {
-      emit layerWillBeRemoved( myId );
-      emit layerWillBeRemoved( lyr );
       delete lyr;
       mOwnedLayers.remove( lyr );
     }