`Nullable!C.nullify` no longer calls .destroy when `C` is a class or interface

Previously, when `.nullify` is called on a `Nullable!C` where `C` is a class or
interface, the underlying object is destructed immediately via the `.destroy`
function. This led to bugs when there are still references to the object
outside of the `Nullable` instance:

------
class C
{
    int canary = 0xA71FE;
    ~this()
    {
        canary = 0x5050DEAD;
    }
}

auto c = new C;
assert(c.canary == 0xA71FE);

Nullable!C nc = nullable(c);
nc.nullify;
assert(c.canary == 0xA71FE); // This would fail
------

The `.nullify` method has been fixed so that it no longer calls `.destroy` on
class or interface instances, and the above code will now work correctly.