Resolve differences between SystemController::overview and systemAdminMenuBlockPage

Problem/Motivation

While working on #296693: Restrict access to empty top level administration pages discovered that \Drupal\system\Controller\SystemController::systemAdminMenuBlockPage and \Drupal\system\Controller\SystemController::overview do very similar things. This issue is to resolve the differences between the two so we can remove one of them since there are only 2 usages of \Drupal\system\Controller\SystemController::overview in system.routing.yml.

Steps to reproduce

Uninstall node module (first you have to uninstall history and taxonomy)
Go to /admin/content

Proposed resolution

Remove \Drupal\system\Controller\SystemController::overview after making sure systemAdminMenuBlockPage does everything overview was doing and replace the 2 usages of overview in system.routing.yml with systemAdminMenuBlockPage instead.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet