[Documentation] Removal of sitemap import in installation

Author: lchrusciel

doc/installation.md

@@ -54,16 +54,6 @@ bitbag_sylius_cms_plugin:
     resource: "@BitBagSyliusCmsPlugin/Resources/config/routing.yml"
 ```
 
-Import optional sitemap providers:
-```yaml
-# config/services.yaml
-...
-imports:
-...
-    - { resource: "@BitBagSyliusCmsPlugin/Resources/config/services/sitemap_provider.yml" }
-```
-
-
 Finish the installation by updating the database schema and installing assets:
 ```
 $ bin/console doctrine:migrations:diff