Fix offline editing (#3620)

We cannot just restore all layer properties, offline editing relies on some not
being copied.

src/core/qgsvectorlayer.cpp

@@ -1994,7 +1994,7 @@ bool QgsVectorLayer::readSymbology( const QDomNode& node, QString& errorMessage
 
   mConditionalStyles->readXml( node );
 
-  readCustomProperties( node );
+  readCustomProperties( node, "variable" );
 
   return true;
 }