Remove instances of deprecated Snippet code calls

What does this MR do?

Refs #214643 (closed)

Removes instances of the deprecated code attribute for Snippets from the Project Snippets API.

This was deprecated ~8 months ago and can now be removed entirely.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.