Remove conversation URI (#11423)

It is not part of ActivityPub and will free up a lot of space

app/models/conversation.rb

@@ -4,17 +4,10 @@
 # Table name: conversations
 #
 #  id         :bigint(8)        not null, primary key
-#  uri        :string
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
 #
 
 class Conversation < ApplicationRecord
-  validates :uri, uniqueness: true, if: :uri?
-
   has_many :statuses
-
-  def local?
-    uri.nil?
-  end
 end