Remove conversation URI (#11423)

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

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_07_26_175042) do
+ActiveRecord::Schema.define(version: 2019_07_28_084117) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -203,10 +203,8 @@ ActiveRecord::Schema.define(version: 2019_07_26_175042) do
   end
 
   create_table "conversations", force: :cascade do |t|
-    t.string "uri"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.index ["uri"], name: "index_conversations_on_uri", unique: true
   end
 
   create_table "custom_emoji_categories", force: :cascade do |t|