tomahawk: Mark as unmaintained by upstream

The upstream commit tomahawk-player/tomahawk@c8389592488c07079f40e7edb91
changed the project's README to state that the project has been
abandoned, quote:

   This project is essentially abandoned

   There is noone working on it. There isn't much sense in adding any
   new issues in the issue tracker unless you want to fix them yourself.

So I'm adding a note in the description and essentially remove my
maintainership as well.

Signed-off-by: aszlig <aszlig@nix.build>