[GUI.Qt] Some cleaning in qt RealGui (#4641)* [QtGui] Remove m_createViewersOpt* [QtGui] Store SofaViewer* directly to avoid multiple dynamic casts* [QtGui] Remove m_isEmbeddedViewer option to simplify code* Apply suggestions from code reviewCo-authored-by: Frederick Roy <fredroy@users.noreply.github.com>* Update RealGUI.cpp---------Co-authored-by: Frederick Roy <fredroy@users.noreply.github.com>