Postgres SERIAL is not a post-insert identifier generation strategy

Blah, nvm, my fault, found that [`Doctrine\ORM\Mapping\ClassMetadataFactory`](https://github.com/doctrine/doctrine2/blob/9280a4621df04bcd625da2a111bb12bd5fda9260/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L512) is `true` for [`Doctrine\DBAL\Platforms\PostgreSqlPlatform`](https://github.com/doctrine/dbal/blob/e35ce0486a1d161e739bed38fde27b7cc921f5f0/lib/Doctrine/DBAL/Platforms/PostgreSqlPlatform.php#L209-L215), so [the sequence name is extracted](https://github.com/doctrine/doctrine2/blob/9280a4621df04bcd625da2a111bb12bd5fda9260/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L540-L544) in any case when using `SERIAL` columns (and `ClassMetadata::GENERATOR_TYPE_AUTO`), and [a sequence generator is used](https://github.com/doctrine/doctrine2/blob/9280a4621df04bcd625da2a111bb12bd5fda9260/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L577-L580).

Merging.