Remove visit_id from the debates table

This was added in commit 02f19aa4b, before we started tracking events.
I don't think we ever used it; in any case, we now use the `Ahoy::Chart`
class to deal with the stats Ahoy used to generate.

app/models/debate.rb

@@ -46,8 +46,6 @@ class Debate < ApplicationRecord
   scope :featured,                 -> { where.not(featured_at: nil) }
   scope :public_for_api,           -> { all }
 
-  visitable class_name: "Visit"
-
   attr_accessor :link_required
 
   def self.recommendations(user)

db/migrate/20240424002959_remove_visit_id_from_debates.rb

@@ -0,0 +1,5 @@
+class RemoveVisitIdFromDebates < ActiveRecord::Migration[7.0]
+  def change
+    remove_column :debates, :visit_id, :string
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2024_03_22_223950) do
+ActiveRecord::Schema[7.0].define(version: 2024_04_24_002959) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "plpgsql"
@@ -528,7 +528,6 @@ ActiveRecord::Schema[7.0].define(version: 2024_03_22_223950) do
     t.integer "author_id"
     t.datetime "created_at", precision: nil, null: false
     t.datetime "updated_at", precision: nil, null: false
-    t.string "visit_id"
     t.datetime "hidden_at", precision: nil
     t.integer "flags_count", default: 0
     t.datetime "ignored_flag_at", precision: nil