Mark unused&immature activitypub as "not implemented" (#36789)

After many years, "activitypub" is still "in progress" and no real
progress for end users. So it is not mature.

Temporarily mark the endpoints as "501 not implemented",
and wait until the whole design is stable and usable.

modules/structs/activitypub.go

@@ -1,10 +0,0 @@
-// Copyright 2022 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package structs
-
-// ActivityPub type
-type ActivityPub struct {
-	// Context defines the JSON-LD context for ActivityPub
-	Context string `json:"@context"`
-}