Removed unused dropdown

What does this MR do?

Removes unused and visually broken dropdown toggle from single-file editor. This dropdown hasn't been really connected to any functionality for a pretty long time so it's time to get rid of it

Screenshots (strongly suggested)

Before	After

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

Related to #296645 (closed)