Hide update_plugin_file impl detail

This is currently automatically performed as part of `merge_delta` so
don't expose it, so nobody tries to call it again.

crates/nu-protocol/src/engine/engine_state.rs

@@ -502,7 +502,7 @@ impl EngineState {
     }
 
     #[cfg(feature = "plugin")]
-    pub fn update_plugin_file(
+    fn update_plugin_file(
         &self,
         updated_items: Vec<PluginRegistryItem>,
     ) -> Result<(), ShellError> {