General: Remove unused variable in wp_unique_filename().

Follow-up to [51653].

Props Soean.
Fixes #64675.

git-svn-id: https://develop.svn.wordpress.org/trunk@61710 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/functions.php

@@ -2556,7 +2556,6 @@ function _wp_upload_dir( $time = null ) {
 function wp_unique_filename( $dir, $filename, $unique_filename_callback = null ) {
 	// Sanitize the file name before we begin processing.
 	$filename = sanitize_file_name( $filename );
-	$ext2     = null;
 
 	// Initialize vars used in the wp_unique_filename filter.
 	$number        = '';