Remove experimental namespace and prefix from libYARP_robotinterface use

[traversaro]

Without this, the build is failing with error (see https://github.com/robotology/robotology-superbuild/runs/3014928864?check_suite_focus=true)

[4:42 PM] Silvio Traversaro
    
2021-07-08T13:18:43.7169616Z /home/runner/work/robotology-superbuild/robotology-superbuild/src/GazeboYARPPlugins/plugins/robotinterface/src/GazeboYarpRobotInterface.cc:27:94: error: 'ActionPhaseInterrupt1' is not a member of 'yarp::robotinterface::experimental'; did you mean 'yarp::robotinterface::ActionPhaseInterrupt1'?
2021-07-08T13:18:43.7172098Z    27 |     bool ok = m_xmlRobotInterfaceResult.robot.enterPhase(yarp::robotinterface::experimental::ActionPhaseInterrupt1);
2021-07-08T13:18:43.7173202Z       |                                                                                              ^~~~~~~~~~~~~~~~~~~~~
2021-07-08T13:18:43.7174381Z In file included from /home/runner/work/robotology-superbuild/robotology-superbuild/build/install/include/yarp/robotinterface/Action.h:12,
2021-07-08T13:18:43.7176079Z                  from /home/runner/work/robotology-superbuild/robotology-superbuild/build/install/include/yarp/robotinterface/Device.h:12,
2021-07-08T13:18:43.7177636Z                  from /home/runner/work/robotology-superbuild/robotology-superbuild/build/install/include/yarp/robotinterface/Robot.h:13,
2021-07-08T13:18:43.7179221Z                  from /home/runner/work/robotology-superbuild/robotology-superbuild/build/install/include/yarp/robotinterface/XMLReader.h:13,
2021-07-08T13:18:43.7180902Z                  from /home/runner/work/robotology-superbuild/robotology-superbuild/build/install/include/yarp/robotinterface/experimental/XMLReader.h:19,
2021-07-08T13:18:43.7182881Z                  from /home/runner/work/robotology-superbuild/robotology-superbuild/src/GazeboYARPPlugins/plugins/robotinterface/include/gazebo/GazeboYarpRobotInterface.hh:12,
2021-07-08T13:18:43.7185007Z                  from /home/runner/work/robotology-superbuild/robotology-superbuild/src/GazeboYARPPlugins/plugins/robotinterface/src/GazeboYarpRobotInterface.cc:7:
2021-07-08T13:18:43.7187359Z /home/runner/work/robotology-superbuild/robotology-superbuild/build/install/include/yarp/robotinterface/Types.h:49:5: note: 'yarp::robotinterface::ActionPhaseInterrupt1' declared here
2021-07-08T13:18:43.7188621Z    49 |     ActionPhaseInterrupt1,
2021-07-08T13:18:43.7189061Z       |     ^~~~~~~~~~~~~~~~~~~~~
2021-07-08T13:18:43.7191343Z /home/runner/work/robotology-superbuild/robotology-superbuild/src/GazeboYARPPlugins/plugins/robotinterface/src/GazeboYarpRobotInterface.cc:31:89: error: 'ActionPhaseShutdown' is not a member of 'yarp::robotinterface::experimental'; did you mean 'yarp::robotinterface::ActionPhaseShutdown'?
2021-07-08T13:18:43.7193734Z    31 |     ok = m_xmlRobotInterfaceResult.robot.enterPhase(yarp::robotinterface::experimental::ActionPhaseShutdown);

[traversaro]

This fixes the Unstable branches of the robotology-superbuild.