Drop deleted testing dir from digging deeper toc

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

Author: ChristophWurst

developer_manual/digging_deeper/index.rst

@@ -20,7 +20,6 @@ Digging deeper
    rest_apis
    search
    settings
-   testing
    two-factor-provider
    users
    dashboard