Deprecated function: Creation of dynamic property FormProcessorWebformHandler::$isPreparing is deprecated in WebformSubmissionHandler->alterForm()

Problem/Motivation

The following deprecation warning is triggered after submitting a form with a CMRF handler:

Deprecated function: Creation of dynamic property Drupal\cmrf_form_processor\Plugin\WebformHandler\FormProcessorWebformHandler::$isPreparing is deprecated in Drupal\cmrf_form_processor\WebformSubmissionHandler->alterForm() (line 78 of modules/contrib/cmrf_form_processor/src/WebformSubmissionHandler.php).

Proposed resolution

That property doesn't seem to be used, let's remove it.