Remove performed_at column

This column is no longer being used so it can be safely removed.

Jira-Issue: MAV-3158

Author: thomasleese

app/models/vaccination_record.rb

@@ -75,8 +75,6 @@
 #  fk_rails_...  (vaccine_id => vaccines.id)
 #
 class VaccinationRecord < ApplicationRecord
-  self.ignored_columns = %w[performed_at]
-
   include BelongsToProgramme
   include Confirmable
   include Discard::Model

db/migrate/20260123154412_remove_performed_at_from_vaccination_records.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class RemovePerformedAtFromVaccinationRecords < ActiveRecord::Migration[8.1]
+  def change
+    remove_column :vaccination_records, :performed_at, :datetime
+  end
+end

db/schema.rb

@@ -945,7 +945,6 @@
     t.integer "outcome", null: false
     t.bigint "patient_id", null: false
     t.jsonb "pending_changes", default: {}, null: false
-    t.datetime "performed_at"
     t.date "performed_at_date", null: false
     t.time "performed_at_time"
     t.string "performed_by_family_name"