When using the node system we have a checkbox on the edit page to deactivate a log field. For custom entities we need to do it ourselves. To your modules/mymodule.module file add:

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(array &$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'MYENTITY_add_form') {
    $form['revision_log']['#access'] = FALSE;
  }
}