Nearly remove NotificationSectionsFeatureManager

This class used to provide a filtered list of sections (and the resulting count) but it has not done anything since the PriorityPeopleSection flag launched, which made the list of sections it returned static.

Change-Id: If8843faff0a5659b81eddbb9cbe4bd5e9badf8e2
Flag: com.android.systemui.priority_people_section
Bug: 340294566
Test: atest NotificationSectionsManagerTest