[IMP] iap: remove documentation related to IAP services creation

It has been decided to remove the possibility for our users to create
their own IAP services.
This decision was motivated by the fact that, in the 5 years of
existence of IAP, very few non-Odoo services have been created and
actively used by our customers.

X-original-commit: 282f041ed1
Part-of: odoo/documentation#6324

content/developer/howtos.rst

@@ -18,7 +18,6 @@ How-to guides
     howtos/accounting_localization
     howtos/translations
     howtos/website_themes
-    howtos/provide_iap_services
     howtos/connect_device
 
 .. cards::
@@ -74,11 +73,6 @@ How-to guides
 
       Learn how to customize your website by creating a custom theme.
 
-   .. card:: Provide IAP services
-      :target: howtos/provide_iap_services
-
-       Learn how to provide ongoing services with Odoo's In-App Purchase (IAP).
-
    .. card:: Connect with a device
       :target: howtos/connect_device