Removed unused variable in Shapes plugin (#984)

Signed-off-by: ahcorde <[email protected]>

src/gui/plugins/shapes/Shapes.cc

@@ -40,9 +40,6 @@ namespace ignition::gazebo
     /// \brief Ignition communication node.
     public: transport::Node node;
 
-    /// \brief Mutex to protect mode
-    public: std::mutex mutex;
-
     /// \brief Transform control service name
     public: std::string service;
   };