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/plugin/api.go

@@ -1158,13 +1158,7 @@ type API interface {
 	// Minimum server version: 7.1
 	EnsureBotUser(bot *model.Bot) (string, error)
 
-	// RegisterCollectionAndTopic informs the server that this plugin handles
-	// the given collection and topic types.
-	//
-	// It is an error for different plugins to register the same pair of types,
-	// or even to register a new topic against another plugin's collection.
-	//
-	// EXPERIMENTAL: This API is experimental and can be changed without advance notice.
+	// RegisterCollectionAndTopic is no longer supported.
 	//
 	// Minimum server version: 7.6
 	RegisterCollectionAndTopic(collectionType, topicType string) error