Remove redundant PreviousSubStatus assignment in workflow context

The PreviousSubStatus assignment was removed as it was unused and unnecessary. This cleanup simplifies the code and ensures focus only on relevant functionality.

src/modules/Elsa.Workflows.Core/Contexts/WorkflowExecutionContext.cs

@@ -513,8 +513,7 @@ public partial class WorkflowExecutionContext : IExecutionContext
     {
         if (!ValidateStatusTransition())
             throw new($"Cannot transition from {SubStatus} to {subStatus}");
-
-        PreviousSubStatus = SubStatus;
+        
         SubStatus = subStatus;
         UpdatedAt = SystemClock.UtcNow;