Remove plugin hooks feature flag (#22552)

# Why

This is a follow-up stacked on top of the `plugin_hooks` default-on
change. Once we are comfortable making plugin hooks part of the normal
plugin behavior, the separate feature flag stops buying us much and
leaves extra branching/cache state behind.

# What

- remove the `PluginHooks` feature and generated config-schema entries
- make plugin hook loading/listing follow plugin enablement directly
- drop plugin-manager cache/state that only existed to distinguish
hook-flag toggles
- remove tests and fixtures that modeled `plugin_hooks = true/false`

codex-rs/core-plugins/src/test_support.rs

@@ -120,11 +120,6 @@ pub(crate) async fn load_plugins_config(codex_home: &Path, cwd: &Path) -> Plugin
             "remote_plugin",
             /*default_enabled*/ false,
         ),
-        feature_enabled(
-            &effective_config,
-            "plugin_hooks",
-            /*default_enabled*/ false,
-        ),
         "https://chatgpt.com/backend-api/".to_string(),
     )
 }