[FEM] add missing functions deletion handling

as it was, one could delete the functions container breaking the simulation. Now you get a warning.

- also remove unused includes

src/Mod/Fem/Gui/ViewProviderAnalysis.cpp

@@ -33,13 +33,11 @@
 
 #include <App/MaterialObject.h>
 #include <App/TextDocument.h>
-
 #include <Gui/ActionFunction.h>
 #include <Gui/Command.h>
 #include <Gui/Control.h>
 #include <Gui/Document.h>
 #include <Gui/MainWindow.h>
-
 #include <Mod/Fem/App/FemAnalysis.h>
 #include <Mod/Fem/App/FemConstraint.h>
 #include <Mod/Fem/App/FemMeshObject.h>