Kumar,
Item category is assigned in customizing. The path is:
Sales and Distribution - Sales - Sales Documents - Sales Document Item - Assign Item Categories
Here on the basis of sales order document type, item category group & item usage combination we assign various item categories. Cross check in your system and you should see these two or more (as maintained in your system).
Though its not clear why you want to do this but before you remove this, cross check you don't have open orders and test thoroughly.
Regards
Rahul