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-protocol/schema/json/ClientRequest.json

@@ -2894,7 +2894,6 @@
         "vscode",
         "exec",
         "appServer",
-        "custom",
         "subAgent",
         "subAgentReview",
         "subAgentCompact",