Always skip children when using Plain write mode

Outputting child module contents in the "Plain" write mode does not make
sense, since there is no way to differentiate code that came from a
child module from that which came from the parent file.

src/lib.rs

@@ -280,8 +280,11 @@ fn format_ast(krate: &ast::Crate,
               config: &Config)
               -> FileMap {
     let mut file_map = FileMap::new();
+    // We always skip children for the "Plain" write mode, since there is
+    // nothing to distinguish the nested module contents.
+    let skip_children = config.skip_children || config.write_mode == config::WriteMode::Plain;
     for (path, module) in modules::list_files(krate, parse_session.codemap()) {
-        if config.skip_children && path.as_path() != main_file {
+        if skip_children && path.as_path() != main_file {
             continue;
         }
         let path = path.to_str().unwrap();