Remove OPENAI_BASE_URL config fallback (#16720)

The `OPENAI_BASE_URL` environment variable has been a significant
support issue, so we decided to deprecate it in favor of an
`openai_base_url` config key. We've had the deprecation warning in place
for about a month, so users have had time to migrate to the new
mechanism. This PR removes support for `OPENAI_BASE_URL` entirely.

codex-rs/tui/tests/suite/model_availability_nux.rs

@@ -91,7 +91,6 @@ trust_level = "trusted"
         .env("CODEX_HOME", codex_home.path())
         .env("OPENAI_API_KEY", "dummy")
         .env("CODEX_RS_SSE_FIXTURE", fixture_path)
-        .env("OPENAI_BASE_URL", "http://unused.local")
         .output()
         .context("failed to execute codex exec")?;
     anyhow::ensure!(