Remove legacy field in the visits table

The search_keyword column was removed for new installations in Ahoy
2.1.0 and stopped being used completely in Ahoy 3.0.0.

There are other columns that we use and are no longer generated by
default by Ahoy, which are: screen_height, screen_width and postal_code.
Apparently they're still used in Ahoy if they're available, though, so
we aren't removing them, at least for now.

db/migrate/20241026111636_remove_search_keyword_from_visits.rb

@@ -0,0 +1,5 @@
+class RemoveSearchKeywordFromVisits < ActiveRecord::Migration[7.0]
+  def change
+    remove_column :visits, :search_keyword, :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_05_13_135357) do
+ActiveRecord::Schema[7.0].define(version: 2024_10_26_111636) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "plpgsql"
@@ -1686,7 +1686,6 @@ ActiveRecord::Schema[7.0].define(version: 2024_05_13_135357) do
     t.text "landing_page"
     t.integer "user_id"
     t.string "referring_domain"
-    t.string "search_keyword"
     t.string "browser"
     t.string "os"
     t.string "device_type"