Get rid of menu_list_system_menus()

Issue Summary

Follow-up from #1814916-54: Convert menus into entities

We have a inconsistency in menus, some of them could be renamed/deleted and some not.

Proposed solution
Move the Menu entity type into Menu module and thus, detaching the entire Menu UI business logic from the low-level {menu_links}.menu_name container concept.

@sun thoughts:

However, in thinking through this issue over the past days, I wondered whether we need the Menu entity at all, and whether we cannot simply move the entire code for menus into Menu module. As a result of doing so, you would no longer see any menu blocks when Menu module is disabled. (Why would anyone expect any menu to appear without it anyway?) — The router/menu link building should not be affected by that; it can still operate on the 'menu_name' property of menu links. Essentially, we'd turn the 'menu_name' property of menu links into a simple "container"/realm that has no further business logic and also no UI attached to it by default. Only by enabling Menu module, those containers/realms are actually turned into something that has business logic behind it.