[REM] ecommerce: removed upsell,cross-sell, and promo code

Remove unnecessary, straightforward, easy-to-figure out documentation, as part of an effort to clear
out any sort of documentation that is either too easy, too simple,
or already gone over in the tutorial videos. This documentation falls into that category. Hence,
it was removed entirely.

Author: ksc-odoo

ecommerce/maximizing_revenue.rst

@@ -5,11 +5,9 @@ Maximize my revenue
 .. toctree::
    :titlesonly:
 
-   maximizing_revenue/upselling
-   maximizing_revenue/cross_selling
    maximizing_revenue/reviews
    maximizing_revenue/pricing
-   maximizing_revenue/promo_code
+