Clean up stale branches

[dscho]

When I go to https://github.com/git/git-scm.com/branches/stale, I see the following stale branches:

@pedrorijo91 do you need v2 or revert-1806-anna_skoulikari_add_book, still?

@peff, do you need css-fix, still?

@jnavila, do you need book_svg_images, still?

v2 seems to have 117 unmerged commits, the three other branches seem to have one unmerged commit each.

[peff]

Nope, I'll delete mine.

[dscho]

Nope, I'll delete mine.

Thank you, @peff!

[jnavila]

Done