Unnecessary sink state

[rsmmr]

Given this input:

module Test;

public type Foo = unit {
    sink b;
};

Having the sink in there leads to this struct:

struct Foo : ::hilti::rt::trait::isStruct, ::hilti::rt::Controllable<Foo> {
        ::hilti::rt::StrongReference<::spicy::rt::Sink> b{};
        ::spicy::rt::sink::detail::State* __sink{};
        [...]
    };

__sink seems unnecessary here because there's nothing connecting Foo itself to a sink. Would be nice if we could optimize that out.

Note: This isn't a huge thing. At first I thought we're instantiating the full sink state already but then I realized it's just a pointer left null until there's a connect happening.