"Atom" snowblock decommission (#252)

Atom [1] is not used anymore since at least February 23 2019 and was
replaced by Visual Studio Code [2] in GH-179. Therefore the `atom`
snowblock_ has been removed while the data is still available through
the Git repository history/logs.

[1]: https://atom.io
[2]: https://code.visualstudio.com
[3]: https://git-scm.com/book/en/v2/Git-Basics-Viewing-the-Commit-History

Closes GH-251
Related to GH-248

Co-authored-by: Sven Greb <[email protected]>