SchemaTool generates extra diff for platforms without FK support

[vpArth]

Bug Report

SchemaTool generates extra diff for platforms without FK support(ex: SqlitePlatform)

Q	A
BC Break	no
Version	all

Summary

SchemaTool::getUpdateSchemaSql is never empty, because MetaData contains foreign keys adding, but SqlitePlatform has not support of them

How to reproduce

/** @ORM\Entity */
class A {
    /** @ORM\Id @ORM\Column(type="integer") @ORM\GeneratedValue */
    public $id;
    /** @ORM\OneToMany(targetEntity=B::class, mappedBy="parent") */
    public $children;
}
/** @ORM\Entity */
class B {
    /** @ORM\Id @ORM\Column(type="integer") @ORM\GeneratedValue */
    public $id;
    /** @ORM\ManyToOne(targetEntity=A::class) */
    public $parent;
}
// ...
    $class = $em->getClassMetadata(B::class);
    $schemaTool->updateSchema([$class]);
    $diff = $schemaTool->getUpdateSchemaSql([$class]);

Current behavior

count($diff) > 0;

Expected behavior

count($diff) === 0;

[lcobucci]

Handled by #7842