Fixed file templates not clearing in Web IDE

What does this MR do?

This fixes a bug in the Web IDE file templates where the templates list would contain a list of all previous templates. Now when changing the template type the templates array gets cleared.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52487

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides