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/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