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_test.go

@@ -2019,89 +2019,6 @@ func TestPluginAPIIsEnterpriseReady(t *testing.T) {
 	assert.Equal(t, true, api.IsEnterpriseReady())
 }
 
-func TestRegisterCollectionAndTopic(t *testing.T) {
-	os.Setenv("MM_FEATUREFLAGS_THREADSEVERYWHERE", "true")
-	defer os.Unsetenv("MM_FEATUREFLAGS_THREADSEVERYWHERE")
-	th := Setup(t)
-	defer th.TearDown()
-	th.App.UpdateConfig(func(cfg *model.Config) {
-		cfg.FeatureFlags.ThreadsEverywhere = true
-	})
-	api := th.SetupPluginAPI()
-
-	err := api.RegisterCollectionAndTopic("collection1", "topic1")
-	assert.NoError(t, err)
-	err = api.RegisterCollectionAndTopic("collection1", "topic1")
-	assert.NoError(t, err)
-	err = api.RegisterCollectionAndTopic("collection1", "topic2")
-	assert.NoError(t, err)
-	err = api.RegisterCollectionAndTopic("collection2", "topic3")
-	assert.NoError(t, err)
-	err = api.RegisterCollectionAndTopic("collection2", "topic1")
-	assert.Error(t, err)
-
-	pluginCode := `
-    package main
-
-    import (
-	  "github.com/pkg/errors"
-      "github.com/mattermost/mattermost/server/public/plugin"
-    )
-
-    type MyPlugin struct {
-      plugin.MattermostPlugin
-    }
-
-	func (p *MyPlugin) OnActivate() error {
-		if err := p.API.RegisterCollectionAndTopic("collectionTypeToBeRepeated", "some topic"); err != nil {
-			return errors.Wrap(err, "cannot register collection")
-		}
-		if err := p.API.RegisterCollectionAndTopic("some collection", "topicToBeRepeated"); err != nil {
-			return errors.Wrap(err, "cannot register collection")
-		}
-		return nil
-	}
-
-    func main() {
-      plugin.ClientMain(&MyPlugin{})
-    }
-  `
-	pluginDir, err := os.MkdirTemp("", "")
-	require.NoError(t, err)
-	webappPluginDir, err := os.MkdirTemp("", "")
-	require.NoError(t, err)
-
-	th.App.UpdateConfig(func(cfg *model.Config) {
-		*cfg.PluginSettings.Directory = pluginDir
-		*cfg.PluginSettings.ClientDirectory = webappPluginDir
-	})
-
-	newPluginAPI := func(manifest *model.Manifest) plugin.API {
-		return th.App.NewPluginAPI(th.Context, manifest)
-	}
-
-	env, err := plugin.NewEnvironment(newPluginAPI, NewDriverImpl(th.App.Srv()), pluginDir, webappPluginDir, th.App.Log(), nil)
-	require.NoError(t, err)
-
-	th.App.ch.SetPluginsEnvironment(env)
-
-	pluginID := "testplugin"
-	pluginManifest := `{"id": "testplugin", "server": {"executable": "backend.exe"}}`
-	backend := filepath.Join(pluginDir, pluginID, "backend.exe")
-	utils.CompileGo(t, pluginCode, backend)
-
-	os.WriteFile(filepath.Join(pluginDir, pluginID, "plugin.json"), []byte(pluginManifest), 0600)
-	manifest, activated, reterr := env.Activate(pluginID)
-	require.NoError(t, reterr)
-	require.NotNil(t, manifest)
-	require.True(t, activated)
-
-	err = api.RegisterCollectionAndTopic("collectionTypeToBeRepeated", "some other topic")
-	assert.Error(t, err)
-	err = api.RegisterCollectionAndTopic("some other collection", "topicToBeRepeated")
-	assert.Error(t, err)
-}
-
 func TestPluginUploadsAPI(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()