I am going to describe how to delete unwanted or orphaned workflow column from a List.it can be done simply by writing few lines of powershell command as below.
 
- function Delete-WorkflowColumn ($webURL, $listName, $columnName)  
-  {  
-  #Setup variables from the user input  
-  $web = Get-SPWeb $webURL  
-  $list = $web.Lists[$listName]  
-  $column = $list.Fields[$columnName]   
- #Make sure the column is not hidden or read only  
-  $column.Hidden = $false   
- $column.ReadOnlyField = $false   
- $column.Update()   
- #Delete column and dispose of the web object   
- $list.Fields.Delete($column)   
- write-host "Deleted column"   
- $web.Dispose()   
- }  
-    
-   
- Delete-WorkflowColumn -webURL http://ServerName/ -listName "ERP Request" -columnName   
-   
- "Approval Workflow"  
 
 
 save above notepad with the name of delete.ps1 and then from powershellscript do as below
- PS D:\> .\delete.ps1  
-  Deleted column  
-  PS D:\>