Fix the destructor of `conc::sync::treiber`, which previously double-dropped.
Fix the destructor of `conc::sync::treiber`, which previously double-dropped.

What would happen was that whenever the head was overwritten, the `Drop::drop` method would be called, leading to destruction of the stack all the way through. Now this is only called when the stack itself is removed.

ticki/tfs,redox-os/tfs