MM-15632 DeleteEphemeralMessage to accept a postId string (#10853)

Before it was accepting `post *model.Post`, and returning one.
This is a breaking change for the giphy plugin and any other that rely
on the API.

app/plugin_api.go

@@ -447,8 +447,8 @@ func (api *PluginAPI) UpdateEphemeralPost(userId string, post *model.Post) *mode
 	return api.app.UpdateEphemeralPost(userId, post)
 }
 
-func (api *PluginAPI) DeleteEphemeralPost(userId string, post *model.Post) {
-	api.app.DeleteEphemeralPost(userId, post)
+func (api *PluginAPI) DeleteEphemeralPost(userId, postId string) {
+	api.app.DeleteEphemeralPost(userId, postId)
 }
 
 func (api *PluginAPI) DeletePost(postId string) *model.AppError {

app/post.go

@@ -437,16 +437,18 @@ func (a *App) UpdateEphemeralPost(userId string, post *model.Post) *model.Post {
 	return post
 }
 
-func (a *App) DeleteEphemeralPost(userId string, post *model.Post) *model.Post {
-	post.Type = model.POST_EPHEMERAL
-	post.DeleteAt = model.GetMillis()
-	post.UpdateAt = post.DeleteAt
-	message := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_POST_DELETED, "", post.ChannelId, userId, nil)
+func (a *App) DeleteEphemeralPost(userId, postId string) {
+	post := &model.Post{
+		Id:       postId,
+		UserId:   userId,
+		Type:     model.POST_EPHEMERAL,
+		DeleteAt: model.GetMillis(),
+		UpdateAt: model.GetMillis(),
+	}
 
+	message := model.NewWebSocketEvent(model.WEBSOCKET_EVENT_POST_DELETED, "", "", userId, nil)
 	message.Add("post", post.ToJson())
 	a.Publish(message)
-
-	return post
 }
 
 func (a *App) UpdatePost(post *model.Post, safeUpdate bool) (*model.Post, *model.AppError) {