Unlist bevy_retro from assets due to controversial, likely unusable license.

Currently bevy lists bevy_retro in assets. ([main/Assets/2D/bevy_retro.toml](https://github.com/bevyengine/bevy-assets/blob/main/Assets/2D/bevy_retro.toml))
bevy_retro is licensed under what they call the [Katharos license](https://github.com/katharostech/bevy_retrograde/blob/master/LICENSE.md)

The core issue i have with this license is that very large parts of the limitations it puts on usage are very subjective and up to interpretation. Interpretation of one of the most disputed and re-interpreted texts in human history. Something a user might think the license allows, might be forbidden by the authors interpretation of it. This leads to enormous uncertainty - if i may play devils advocate: the author could turn around and suddenly proclaim that a big project using bevy_retro is infringing on something the author interprets the license to say. This would require a legal battle, which could cost both sides a lot of time and money.

I think keeping something *this* ambigous on a site which could easily be understood as a "official bevy endorsement" is a risk bevy shouldn't be willing to take.

Open Assets/2D/bevy_retro.toml and review the license concerns and existing discussion in issue #229 before making a decision. Done means the bevy_retro entry is no longer listed in the Bevy assets collection if maintainers agree it should be removed.

Written by the indexing model from the issue text.