Don't add "how to clone" description to annex mode siblings

[adswa]

As seen in #160 (not the original issue, but the comment from Feb. 13th), annex mode OSF siblings have the same generic OSF project description. This includes instructions how to clone them. For annex mode, however, we don't deposit anything Git-related, only annex keys - so this mode isn't meant to be cloned (and leads to "empty repositories").

[adswa]

Actually, this doesn't match the documentation. I have to remind myself what the different modes do, and clarify this also in documentation and docstrings.

[adswa]

One can clone in the following modes:

annex, export, gitonly. All of these modes include a description how to clone in the project description.

One can't clone in exportonly mode. Siblings created in this mode do not contain a description how to clone (e.g., https://osf.io/dgauf/).

So there actually isn't an issue like the top comment indicates - just my confusion.