Remove banner image from readme

[jakobharder]

And other images as well for now until we support it.

That change is the last I'd need to use file::README.md instead of the lengthy description within the modinfo.json.

[taubenangriff]

Okay, doesn't our markdown library support images in the description already? Excluding the banner image from the readme is probably a good idea tho.

[jakobharder]

I had the image markdown code in red instead of images. Might be that relative images within a mod aren't supported, but absolute urls are (imya.md does show it, right?)

[taubenangriff]

Ah this could very well be the case. In that case, let's move that to a future issue and merge this PR first.