Remove SM top-nav CSS

[akeller]

Description

When the Self-Managed icon was removed from the top nav (I couldn't find the PR for it 😞), the CSS remained, creating a visually odd experience. An area of the Self-Managed docs also randomly picked up the styles.

Thanks for the nudge @Sijoma!

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

Changes to docs infra, including updates to workflows and adding new npm packages, must be first discussed via issue or #ask-c8-documentation and linked for context.

• My changes require a docs infrastructure review.

[Sijoma]

Fixing CSS by removing it. That's how it always works.