rm single-instrument mode: no need to disable instruments at GUI level

GUI/Model/Project/ProjectDocument.cpp

GUI/Model/Project/ProjectDocument.h

GUI/View/Data/DataView.cpp

GUI/View/Data/DataView.h

GUI/View/Data/DatafilesSelector.cpp

GUI/View/Instrument/InstrumentListing.cpp

GUI/View/Instrument/InstrumentListing.h

GUI/View/Instrument/InstrumentView.cpp

GUI/View/Instrument/InstrumentView.h

GUI/View/Main/ActionManager.cpp

GUI/View/Project/ProjectManager.cpp

GUI/View/Project/SimulationView.cpp

GUI/View/Project/SimulationView.h

GUI/View/Widget/ApplicationSettings.cpp

GUI/View/Widget/ApplicationSettings.h