From your description of the problem, you have received the new Windows 11 start menu, and it is not very configurable yet, there is no setting or method available to remove the all apps section from the new Windows 11 start menu.
Onb your start menu, do you now see blocks of tiles each with 3 apps inside, if so, that is the new WIndows 11 Start Menu, the only setting currently on that menu, is to click the 'Category' drop-down menu and select Grid or List.
The best option is to open the Feedback Hub app in Windows and report the lack of configurable settings on the new Start Menu to their developers there.