Remove ref from snippet files API endpoints

What does this MR do?

In this MR we remove the ref param from the snippet files REST API endpoints. We don't really need that param because snippets are not allowed to have multiples branches nor tags.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content