Remove sddi-social image variant

[BWibo]

This has never been completed and has never been used. Do we really need it @MarijaKnezevic?

[thomashkolbe]

It might become useful again in the future. Does the inclusion of this image cause signficant extra work? If yes, we could also think about hibernating the image and to add an information at appropriate places that there is also the possibility to add public commenting of each dataset using Disqus (with a pointer to the repo), but that this facility is currently not maintained.
@BWibo, @MarijaKnezevic: What do you think?

[MarijaKnezevic]

@BWibo yes true. This function was never used. We can document the idea for this in the "Future Ideas Features" PR, but for now, to reduce maintenance, this can be deleted or mentioned that this image is only going to be updated if this funcionality will be required.

[BWibo]

It's not really causing any extra work, but the extension we use does not seem to be maintained (last commit 2 years ago).
And we never tested if this is actually working. As this is a git repo, we are not losing anything if we remove it for now and leave a note that this feature is generally available.

I suggested this mainly because I would like to sharpen the idea of this repo a bit more clearly.
It should become more clear, that we provide a base image in this repo and we are open to integrate customized images in this repo, that build upon the base image. (see #58).