Remove unused Snippets content_types method

What does this MR do?

Removes what appears to be an unused method Snippets#content_types. When tracing through an issue related to snippet file types, I came across this method which does not appear to be used - this MR is to remove it to reduce confusion.

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done