Remove `InterfaceIsType` checkstyle module

[Randgalt]

This check prevents a modern Java idiom (and likely future idioms). E.g.

public sealed interface Choice
{
    record GoodChoice(...)
        implements Choice
    {
    }

    record BadChoice(...)
        implements Choice
    {
    }
}

The previous purpose of InterfaceIsType is arguably out of date anyway. It's better to use an interface for utility containers, etc.

[wendigo]

@electrum WDYT? I'm in favor of merging it

[wendigo]

Let's give it a shot.