Remove dead code, refs #8517

src/Composer/Command/ConfigCommand.php

@@ -466,16 +466,6 @@ EOT
                 },
             ),
         );
-        $uniqueOrDotNestedArray = array(
-        	'preferred-install' => array(
-		        function ($val) {
-			        return in_array($val, array('auto', 'source', 'dist'), true);
-		        },
-		        function ($val) {
-			        return $val;
-		        },
-	        ),
-        );
 
         if ($input->getOption('unset') && (isset($uniqueConfigValues[$settingKey]) || isset($multiConfigValues[$settingKey]))) {
             if ($settingKey === 'disable-tls' && $this->config->get('disable-tls')) {