Avoid defining a new function

It's less new code to simply define this inline

lib/client-assets.php

@@ -1033,9 +1033,17 @@ function gutenberg_editor_scripts_and_styles( $hook ) {
  */
  $allowed_block_types = apply_filters( 'allowed_block_types', true, $post );
 
+ $available_templates = wp_get_theme()->get_page_templates( $post_to_edit['id'] );
+ $available_templates = array_merge(
+ array(
+ '' => apply_filters( 'default_page_template_title', __( 'Default template', 'gutenberg' ), 'rest-api' ),
+ ),
+ $available_templates
+ );
+
  $editor_settings = array(
  'alignWide' => $align_wide || ! empty( $gutenberg_theme_support[0]['wide-images'] ), // Backcompat. Use `align-wide` outside of `gutenberg` array.
- 'availableTemplates' => gutenberg_get_available_page_attributes_templates( $post_to_edit['id'] ),
+ 'availableTemplates' => $available_templates,
  'allowedBlockTypes' => $allowed_block_types,
  'disableCustomColors' => get_theme_support( 'disable-custom-colors' ),
  'disablePostFormats' => ! current_theme_supports( 'post-formats' ),
@@ -1094,34 +1102,6 @@ function gutenberg_editor_scripts_and_styles( $hook ) {
  do_action( 'enqueue_block_editor_assets' );
 }
 
-/**
- * Get available templates for a post.
- *
- * We wrap the WP_Theme::get_page_templates() method since we wish to support the
- * `default_page_template_title` filter to change the name of the default template.
- *
- * @since TBD
- *
- * @param int $post_id The ID of the post being edited.
- * @return array
- *
- * @see WP_Theme::get_page_templates()
- */
-function gutenberg_get_available_page_attributes_templates( $post_id ) {
- // We only want to add the default template to the array if there are other
- // templates since an array with only the default template will trigger the
- // template select input in the meta box.
- $templates = wp_get_theme()->get_page_templates( get_post( $post_id ) );
- if ( is_array( $templates ) && ! empty( $templates ) ) {
- return array_merge(
- array( '' => apply_filters( 'default_page_template_title', __( 'Default template' ), 'gutenberg' ), ),
- $templates
- );
- } else {
- return array();
- }
-}
-
 /**
  * Ensure the editor module is loaded before third party plugins.
  *