Issue #2080333 by mrsinguyen, lorique: Remove Unused local variable $instance...

Issue #2080333 by mrsinguyen, lorique: Remove Unused local variable $instance from /core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php.

core/modules/system/lib/Drupal/system/Tests/File/ReadOnlyStreamWrapperTest.php

@@ -51,7 +51,7 @@ function testWriteFunctions() {
 
     // Generate a read-only stream wrapper instance
     $uri = $this->scheme . '://' . $filename;
-    $instance = file_stream_wrapper_get_instance_by_scheme($this->scheme);
+    file_stream_wrapper_get_instance_by_scheme($this->scheme);
 
     // Attempt to open a file in read/write mode
     $handle = @fopen($uri, 'r+');