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.

sdk/typescript/tests/exec.test.ts

@@ -134,7 +134,6 @@ describe("CodexExec", () => {
       expect(spawnEnv.CODEX_HOME).toBe("/tmp/codex-home");
       expect(spawnEnv.CUSTOM_ENV).toBe("custom");
       expect(spawnEnv.CODEX_ENV_SHOULD_NOT_LEAK).toBeUndefined();
-      expect(spawnEnv.OPENAI_BASE_URL).toBeUndefined();
       expect(spawnEnv.CODEX_API_KEY).toBe("test");
       expect(spawnEnv.CODEX_INTERNAL_ORIGINATOR_OVERRIDE).toBeDefined();
       expect(commandArgs).toContain("--config");