Remove plan_id from GroupChangesAuditor

plan_id is not used so tracking changes to this column is useless

ee/lib/ee/audit/group_changes_auditor.rb

@@ -7,11 +7,10 @@ module EE
                    request_access_enabled membership_lock lfs_enabled
                    shared_runners_minutes_limit
                    require_two_factor_authentication
-                   two_factor_grace_period plan_id
+                   two_factor_grace_period
                    project_creation_level).freeze
 
       COLUMN_HUMAN_NAME = {
-        plan_id: 'plan',
         visibility_level: 'visibility'
       }.freeze
 
@@ -36,11 +35,6 @@ module EE
             from: ::Gitlab::Access.project_creation_level_name(old),
             to: ::Gitlab::Access.project_creation_level_name(new)
           }
-        when :plan_id
-          {
-            from: plan_name(old),
-            to: plan_name(new)
-          }
         else
           {
             from: old,
@@ -51,12 +45,6 @@ module EE
 
       private
 
-      def plan_name(plan_id)
-        return 'none' unless plan_id.present?
-
-        Plan.find_by_id(plan_id.to_i)&.name || 'unknown'
-      end
-
       def column_human_name(column)
         COLUMN_HUMAN_NAME.fetch(column, column.to_s)
       end

ee/spec/lib/ee/audit/group_changes_auditor_spec.rb

@@ -39,19 +39,6 @@ describe EE::Audit::GroupChangesAuditor do
         expect(event.details[:change]).to eq 'project_creation_level'
       end
 
-      it 'creates an event when the group plan changes' do
-        new_plan = create(:free_plan, name: 'plan-1')
-
-        group.update!(plan_id: new_plan.id)
-
-        expect { foo_instance.execute }.to change { SecurityEvent.count }.by(1)
-
-        event = SecurityEvent.last
-        expect(event.details[:from]).to eq 'none'
-        expect(event.details[:to]).to eq 'plan-1'
-        expect(event.details[:change]).to eq 'plan'
-      end
-
       it 'creates an event when attributes change' do
         # Exclude special cases covered from above
         columns = described_class::COLUMNS - described_class::COLUMN_HUMAN_NAME.keys - [:project_creation_level]