fix(Interaction): ensure valid grab attempt is called

The Interact Grab script requires the `IsValidGrabAttempt` method
to be called to set up state for future method calls.

This was removed during a cleanup that said the line was not being
used, but the variable it was being assigned to wasn't being used,
however, the method call was still required.

Assets/VRTK/Source/Scripts/Interactions/VRTK_InteractGrab.cs

@@ -515,6 +515,7 @@ protected virtual void AttemptGrabObject()
         protected virtual void PerformGrabAttempt(GameObject objectToGrab)
         {
             IncrementGrabState();
+            IsValidGrabAttempt(objectToGrab);
             undroppableGrabbedObject = GetUndroppableObject();
         }