Remove skipped spec of _signup_box.html.haml_spec.rb

What does this MR do and why?

从 skip_specs.yml 中移除 _signup_box.html.haml_spec.rb。

之前跳过此 Spec 的原因是此测试存在一个 Upstream Bug。现在 Upstream 已经将它修复：https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144004

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