Remove obsolete todo (doctrine#7039)

|      Q       |   A
|------------- | -----------
| Type         | improvement
| Fixed issues | N/A

#### Summary

This type has been removed in DBAL 4. Nobody's going to work on that
`@todo`, so let's remove it.

Author: derrabus

src/Types/ArrayType.php

@@ -35,7 +35,6 @@ public function getSQLDeclaration(array $column, AbstractPlatform $platform)
      */
     public function convertToDatabaseValue($value, AbstractPlatform $platform)
     {
-        // @todo 3.0 - $value === null check to save real NULL in database
         return serialize($value);
     }