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/main.rs

@@ -4,7 +4,6 @@ use codex_app_server::run_main_with_transport;
 use codex_arg0::Arg0DispatchPaths;
 use codex_arg0::arg0_dispatch_or_else;
 use codex_core::config_loader::LoaderOverrides;
-use codex_protocol::protocol::SessionSource;
 use codex_utils_cli::CliConfigOverrides;
 use std::path::PathBuf;
 
@@ -22,17 +21,6 @@ struct AppServerArgs {
         default_value = AppServerTransport::DEFAULT_LISTEN_URL
     )]
     listen: AppServerTransport,
-
-    /// Session source stamped into new threads started by this app-server.
-    ///
-    /// Known values such as `vscode`, `cli`, `exec`, and `mcp` map to built-in
-    /// sources. Any other non-empty value is recorded as a custom source.
-    #[arg(
-        long = "session-source",
-        value_name = "SOURCE",
-        default_value = "vscode"
-    )]
-    session_source: String,
 }
 
 fn main() -> anyhow::Result<()> {
@@ -44,8 +32,6 @@ fn main() -> anyhow::Result<()> {
             ..Default::default()
         };
         let transport = args.listen;
-        let session_source = SessionSource::from_startup_arg(args.session_source.as_str())
-            .map_err(|err| anyhow::anyhow!("invalid --session-source: {err}"))?;
 
         run_main_with_transport(
             arg0_paths,
@@ -53,7 +39,6 @@ fn main() -> anyhow::Result<()> {
             loader_overrides,
             /*default_analytics_enabled*/ false,
             transport,
-            session_source,
         )
         .await?;
         Ok(())