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.

models/user/setting_options.go

@@ -11,10 +11,6 @@ const (
 	// SettingsKeyShowOutdatedComments is the setting key whether or not to show outdated comments in PRs
 	SettingsKeyShowOutdatedComments = "comment_code.show_outdated"
 
-	// UserActivityPubPrivPem is user's private key
-	UserActivityPubPrivPem = "activitypub.priv_pem"
-	// UserActivityPubPubPem is user's public key
-	UserActivityPubPubPem = "activitypub.pub_pem"
 	// SignupIP is the IP address that the user signed up with
 	SignupIP = "signup.ip"
 	// SignupUserAgent is the user agent that the user signed up with