Resolve #3152940 "Remove uniqid field"

Closes #3152940

src/SerialSQLStorage.php

@@ -90,9 +90,8 @@ class SerialSQLStorage implements ContainerInjectionInterface, SerialStorageInte
 
     try {
       // Insert a temporary record to get a new unique serial value.
-      $uniqid = uniqid('', TRUE);
       $sid = $connection->insert($storageName)
-        ->fields(['uniqid' => $uniqid])
+        ->fields(['sid' => NULL])
         ->execute();
 
       // If there's a reason why it's come back undefined, reset it.
@@ -143,19 +142,8 @@ class SerialSQLStorage implements ContainerInjectionInterface, SerialStorageInte
           'unsigned' => TRUE,
           'description' => 'The atomic serial field.',
         ],
-        'uniqid' => [
-          'type' => 'varchar',
-          'length' => 23,
-          'default' => '',
-          'not null' => TRUE,
-          // @todo review UUID instead
-          'description' => 'Unique temporary allocation Id.',
-        ],
       ],
       'primary key' => ['sid'],
-      'unique keys' => [
-        'uniqid' => ['uniqid'],
-      ],
     ];
     return $schema;
   }