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/channels/app/plugin_api.go

@@ -1232,10 +1232,9 @@ func (api *PluginAPI) GetCloudLimits() (*model.ProductLimits, error) {
 	return limits, err
 }
 
-// RegisterCollectionAndTopic informs the server that this plugin handles
-// the given collection and topic types.
+// RegisterCollectionAndTopic is no longer supported.
 func (api *PluginAPI) RegisterCollectionAndTopic(collectionType, topicType string) error {
-	return api.app.RegisterCollectionAndTopic(api.id, collectionType, topicType)
+	return nil
 }
 
 func (api *PluginAPI) CreateUploadSession(us *model.UploadSession) (*model.UploadSession, error) {