它是由引导4主题.

It is caused by one of the default form themes of Symfony, I think by default currently it's the bootstrap 4 theme.

要在整个应用中消除该类，您可以更改默认表单主题，例如形成div布局:

To get rid of that class across the whole app you can change the default form theme, e.g. to form div layout:

# config/packages/twig.yaml
twig:
    form_themes: ['form_div_layout.html.twig']
    # ...

文档还描述了如何在较小范围内删除该类.

The documentation also describes how to remove the class in smaller scope.