[PHP8.2] Remove undeclared property

[eileenmcnaughton]

It appears to be copy & paste cos it is set but never used

api_v3_ReportTemplateTest::testReportTemplateGetRowsAllReports with data set #29 ('event/incomesummary')
Failure in api call for report_template getrows: Creation of dynamic property CRM_Report_Form_Event_IncomeCountSummary::$_participantWhere is deprecated
#0 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/CRM/Report/Form/Event/IncomeCountSummary.php(209): PHPUnit\Util\ErrorHandler->__invoke(8192, 'Creation of dyn...', '/home/homer/bui...', 209)
#1 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/CRM/Report/Form.php(3086): CRM_Report_Form_Event_IncomeCountSummary->where()
#2 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/api/v3/ReportTemplate.php(146): CRM_Report_Form->buildQuery()
#3 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/api/v3/ReportTemplate.php(105): _civicrm_api3_report_template_getrows(Array)
#4 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/Civi/API/Provider/MagicFunctionProvider.php(89): civicrm_api3_report_template_getrows(Array)
#5 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/Civi/API/Kernel.php(158): Civi\API\Provider\MagicFunctionProvider->invoke(Array)
#6 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/Civi/API/Kernel.php(81): Civi\API\Kernel->runRequest(Array)
#7 /home/homer/buildkit/build/build-1/web/sites/all/modules/civicrm/api/api.php(22): Civi\API\Kernel->runSafe('report_template', 'getrows', Array)

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