You probably want to get rid of Jolteon, even though you did say you wouldn't get rid of it. It's super effective against a few battles, like Skyla, Marlon, and part of Iris' team, but it's rather frail and doesn't learn a lot of good moves. Both Skyla and Marlon are pushovers anyways, so Jolteon isn't truly needed. Golduck gets a solid range of moves, meaning it can contribute more.
There just aren't any other good contenders to be eliminated, as all your Pokemon serve a purpose. You could use Tranquill/Altaria exclusively for Fly though, and only use it when necessary -- thus allowing you to keep all the Pokemon you want to keep.
Golduck @ anything
- Ice Beam
- Strength / Waterfall / anything
Hope I helped!