[REM] purchase: remove dropshipping doc for V14

To reduce the technical debt, we remove easy flows from the documentation.
For dropshipping, it is only a matter of enabling one app setting and ticking
dropship on a product. In addition, dropshipping doesn't really fit under purchase
as it's a way to sell products - that is why it is explained on Odoo Learn under
the Sales app.

purchase/advanced.rst

@@ -5,5 +5,4 @@ Advanced
 .. toctree::
    :titlesonly:
 
-   advanced/dropshipping
    advanced/analyze

purchase/advanced/dropshipping.rst

@@ -1,54 +0,0 @@
-============
-Dropshipping
-============
-
-Dropshipping is a method in which the vendor does not keep products in
-stock but instead transfers the products directly from the supplier to
-the customer.
-
-.. note::
-   You need the *Sales*, *Inventory* and *Purchases* modules
-   for this feature to work.
-
-Activate the dropshipping feature
-=================================
-
-Go to :menuselection:`Purchases --> Configuration --> Settings` and
-select *Dropshipping*.
-
-.. image:: media/dropshipping01.png
-   :align: center
-
-A new route *Dropshipping* has been created. You can go on a product
-and tick the dropshipping route. You also need to define to which vendor
-you will buy the product. Now, each time this product will be sold, it
-will be drop shipped.
-
-.. image:: media/dropshipping02.png
-   :align: center
-
-If you don't always dropship that product, you can go to
-:menuselection:`Sales --> Configuration --> Settings` and tick the box
-in front of *Order-Specific Routes*.
-
-.. image:: media/dropshipping03.png
-   :align: center
-
-When you create a quotation or sales order, you can now decide, order
-line per order line, which products should be drop shipped by selecting
-the dropship route on the SO line.
-
-.. image:: media/dropshipping04.png
-   :align: center
-
-Drop ship a product
-===================
-
-Once a sale order with products to dropship is confirmed, a request for
-quotation is generated by Odoo to buy the product to the vendor. If you
-confirm that request for quotation into a purchase order, it will create
-a transfer from your vendor directly to your customer. The products
-doesn't go through your own stock.
-
-.. image:: media/dropshipping05.png
-   :align: center