Revert "fix: harden plugin feature gating" (#15102)

Reverts openai/codex#15020

I messed up the commit in my PR and accidentally merged changes that
were still under review.

codex-rs/app-server/src/in_process.rs

@@ -808,10 +808,6 @@ mod tests {
         for (requested_source, expected_source) in [
             (SessionSource::Cli, ApiSessionSource::Cli),
             (SessionSource::Exec, ApiSessionSource::Exec),
-            (
-                SessionSource::Custom("atlas".to_string()),
-                ApiSessionSource::Custom("atlas".to_string()),
-            ),
         ] {
             let client = start_test_client(requested_source).await;
             let response = client