Remove dependency on bevy_audio feature

[Salzian]

We previously included the bevy_audio feature to access the AudioSinkPlayback trait. I don't know anymore why I did that. I have no use case and no-one else has voiced a use case. I guess my intention was compatibility, but when using FMOD, my guess is that most users replace their entire audio solution anyways.

In case we need some sort of compatibility, we should find a different solution in the future, i.e. an adapter between bevy_fmod and bevy_audio but as a separate struct.

This had the side effect, that in a project that doesn't use the bevy_audio feature, it still was implicitly introduced, and its structs exposed by bevy::prelude:*.

As bevy_fmod uses some names for its structs that are also present in bevy_audio. This creates a name clash that can currently only be resolved with explicit import or import aliasing.

Closes #85

[GitGhillie]

On vacation right now so I can't test it but it looks good to me. I'll leave an approval but if you want me to test it that will take a week or two