Don’t recursively sanitize child nodes when called from Sanitize

What does this MR do and why?

When using Gitlab::Utils::SanitizeNodeLink.sanitize_unsafe_links from our SanitizationFilter, which uses Sanitize, it automatically calls it for every node. We don't need to do an additional sanitize on each node's children.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.