Remove Releaseable trait again

[tgolsson]

Describe the bug

The Releaseable trait does not work as it's supposed to w.r.t. inheritance and callbacks, which leads to fully correct code, but not necessarily expected behaviors.

Me and @MaikKlein removed the trait to prevent this from causing issues until we find a better solution, in issue #7 and PR #8 (commit 4902bc5). However, it was accidentally diffed the wrong way and added back here when comparing the internal project with this repository, in commit 0f30263.