MM-53358: Revert Threads Everywhere (#23882)

Remove changes related to the unshipped threads everywhere feature, including commits b8da473da7 and 9f9e19e05d.

Since a version of Playbooks shipped calling this experimental API, keep a `nil` implementation to avoid breaking compatibility. We remove the hooks altogether, but keep the numbering again to avoid breaking compatbility.

Fixes: https://mattermost.atlassian.net/browse/MM-53358

server/public/model/collection.go

@@ -1,20 +0,0 @@
-// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
-// See LICENSE.txt for license information.
-
-package model
-
-type CollectionMetadata struct {
-	Id             string `json:"id"`
-	TeamId         string `json:"team_id"`
-	CollectionType string `json:"collection_type"`
-	Name           string `json:"name"`
-	RelativeURL    string `json:"relative_url"`
-}
-
-type TopicMetadata struct {
-	Id             string `json:"id"`
-	TeamId         string `json:"team_id"`
-	TopicType      string `json:"topic_type"`
-	CollectionType string `json:"collection_type"`
-	CollectionId   string `json:"collection_id"`
-}

server/public/model/feature_flags.go

@@ -55,8 +55,6 @@ type FeatureFlags struct {
 
 	PeopleProduct bool
 
-	ThreadsEverywhere bool
-
 	GlobalDrafts bool
 
 	OnboardingTourTips bool
@@ -82,7 +80,6 @@ func (f *FeatureFlags) SetDefaults() {
 	f.CallsEnabled = true
 	f.PostPriority = true
 	f.PeopleProduct = false
-	f.ThreadsEverywhere = false
 	f.GlobalDrafts = true
 	f.DeprecateCloudFree = false
 	f.WysiwygEditor = false