Remove Interface is implemented by 1+ Objects validation

At a glance

• Identifier: #459
• Stage: RFC 3 / Accepted
• Champion: @mjmahone
• Latest activity: Advanced to RFC 3 on 2018-10-02
• Spec PR: https://github.com/graphql/graphql-spec/pull/459

Spec PR description

This validation requirement was a really nasty breaking change for existing GraphQL type systems. Additionally, it is in my opinion actively harmful to an iterative development model, where you'd define an interface that a field returns, develop on the client against that interface, then days or weeks later, create one or more types that implement the interface, which old clients can start using immediately.