Remove individual URL overrides for LLM service (#30290)

This PR removes the individual URL overrides for the LLM service.

We initially had `ZED_PREDICT_EDITS_URL` to allow for directing traffic
to the LLM Worker back when there was still the split of the
Collab-based LLM Service and the Cloudflare-based LLM Worker.

But now that all of the LLM functionality has been moved into the
Worker, we can just direct all traffic there.

Release Notes:

- N/A

crates/zeta/src/zeta.rs

@@ -740,18 +740,13 @@ and then another
             let mut did_retry = false;
 
             loop {
-                let request_builder = http_client::Request::builder().method(Method::POST);
-                let request_builder =
-                    if let Ok(predict_edits_url) = std::env::var("ZED_PREDICT_EDITS_URL") {
-                        request_builder.uri(predict_edits_url)
-                    } else {
-                        request_builder.uri(
-                            http_client
-                                .build_zed_llm_url("/predict_edits/v2", &[])?
-                                .as_ref(),
-                        )
-                    };
-                let request = request_builder
+                let request = http_client::Request::builder()
+                    .method(Method::POST)
+                    .uri(
+                        http_client
+                            .build_zed_llm_url("/predict_edits/v2", &[])?
+                            .as_ref(),
+                    )
                     .header("Content-Type", "application/json")
                     .header("Authorization", format!("Bearer {}", token))
                     .header(ZED_VERSION_HEADER_NAME, app_version.to_string())