Remove dependency to ostatus2

Follwoing mastodon#12822

Author: zunda

Gemfile.lock

@@ -769,7 +769,6 @@ DEPENDENCIES
   omniauth (~> 1.9)
   omniauth-cas (~> 1.1)
   omniauth-saml (~> 1.10)
-  ostatus2 (~> 2.0)
   ox (~> 2.11)
   paperclip (~> 6.0)
   paperclip-av-transcoder (~> 0.6)