Remove Unused local variable $name_field from /core/modules/system/lib/Drupal/system/Tests/Database/SelectOrderedTest.php

Part of meta-issue #2002650: [meta] improve maintainability by removing unused local variables

File /core/modules/system/lib/Drupal/system/Tests/Database/SelectOrderedTest.php

Line 30: Unused local variable $name_field
Line 51: Unused local variable $name_field
Line 82: Unused local variable $name_field