Clean up slash commands VS code extension differentiation method

What does this MR do and why?

Clean up the method for differentiating VS Code requests, now that we’ve switched to using the platform_origin attribute in !158927 (merged).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Connect to VS code with your local GDK instance.
2. Run AI Gateway locally.
3. Type /explain to the Duo Chat in VS Code without selecting any code.
4. The response should be as follows: "Your request does not seem to contain code to explain. To explain code select the lines of code in your editor and then type the command /explain in the chat. You may add additional instructions after this comment. If you have no code to select, you can also simply add the code after the command."

Related to gitlab-vscode-extension#1468 (closed) and its parent issue gitlab-vscode-extension#1215 (closed)