Remove redundant attributes

Required by: https://github.com/D-Programming-Language/dmd/pull/3696
Those returns ALSO have casts. It's pretty obvious that the casts should be removed. An `immutable` or `const` `Variant` should not return a *mutable* reference to itself.

This incorrectly compiles and runs without error with the existing compiler, and likely with your changes as well:

```D
import std.variant;

void main()
{
    immutable v = Variant(5);
    assert(v == 5);
    auto p = v.peek!int();
    assert(p);
    *p = 4;
    assert(v == 4);
}
```

Note: `Variant` really has no good support for being `const` or `immutable`, so I'm not even sure why it has any inout support anyway. I couldn't even print or get the value except (ironically) via `peek`.