Revert "Prevent some unneeded calling of signals"

This reverts commit 85f10e7c4f.
The second time you start Cura, it won't slice due to the number of extruders being wrong.

cura/Settings/ExtruderManager.py

@@ -344,9 +344,7 @@ class ExtruderManager(QObject):
             self._fixSingleExtrusionMachineExtruderDefinition(global_stack)
             if extruders_changed:
                 self.extrudersChanged.emit(global_stack_id)
-
-                # Set it directly instead of using setActiveExtruder, since we want to force the signal to emitted.
-                self._active_extruder_index =  0
+                self.setActiveExtruderIndex(0)
                 self.activeExtruderChanged.emit()
 
     # After 3.4, all single-extrusion machines have their own extruder definition files instead of reusing