Upgrade/Install: Remove the return value of _wp_delete_all_temp_backups().

This function is only utilized as a `shutdown` action callback, so the return value is not used anywhere.

`wp_trigger_error()` is now used instead under the same conditions.

Follow-up to [55720], [56342].

Props johnbillion, amitraj2203, narenin.
Fixes #61116.

git-svn-id: https://develop.svn.wordpress.org/trunk@58906 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/update.php

@@ -1099,8 +1099,6 @@ function wp_delete_all_temp_backups() {
  * @access private
  *
  * @global WP_Filesystem_Base $wp_filesystem WordPress filesystem subclass.
- *
- * @return void|WP_Error Void on success, or a WP_Error object on failure.
  */
 function _wp_delete_all_temp_backups() {
 	global $wp_filesystem;
@@ -1114,15 +1112,17 @@ function _wp_delete_all_temp_backups() {
 	ob_end_clean();
 
 	if ( false === $credentials || ! WP_Filesystem( $credentials ) ) {
-		return new WP_Error( 'fs_unavailable', __( 'Could not access filesystem.' ) );
+		wp_trigger_error( __FUNCTION__, __( 'Could not access filesystem.' ) );
+		return;
 	}
 
 	if ( ! $wp_filesystem->wp_content_dir() ) {
-		return new WP_Error(
-			'fs_no_content_dir',
+		wp_trigger_error(
+			__FUNCTION__,
 			/* translators: %s: Directory name. */
 			sprintf( __( 'Unable to locate WordPress content directory (%s).' ), 'wp-content' )
 		);
+		return;
 	}
 
 	$temp_backup_dir = $wp_filesystem->wp_content_dir() . 'upgrade-temp-backup/';