open_ai: Remove list of supported countries (#29347)

This PR removes the list of supported countries from the `open_ai`
crate, as it is no longer referenced in this repo.

Release Notes:

- N/A

crates/open_ai/src/open_ai.rs

@@ -1,5 +1,3 @@
-mod supported_countries;
-
 use anyhow::{Context as _, Result, anyhow};
 use futures::{
     AsyncBufReadExt, AsyncReadExt, StreamExt,
@@ -15,8 +13,6 @@ use std::{
 };
 use strum::EnumIter;
 
-pub use supported_countries::*;
-
 pub const OPEN_AI_API_URL: &str = "https://api.openai.com/v1";
 
 fn is_none_or_empty<T: AsRef<[U]>, U>(opt: &Option<T>) -> bool {