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/ServerNotification.json

@@ -1880,19 +1880,6 @@
           ],
           "type": "string"
         },
-        {
-          "additionalProperties": false,
-          "properties": {
-            "custom": {
-              "type": "string"
-            }
-          },
-          "required": [
-            "custom"
-          ],
-          "title": "CustomSessionSource",
-          "type": "object"
-        },
         {
           "additionalProperties": false,
           "properties": {