book2.rake: drop "genbook2" task
book2.rake: drop "genbook2" task

We don't actually run this anymore. It imports from Atlas,
which hasn't existed for a few years. Instead, we build
straight from the GitHub repositories with remote_genbook2.

Mokolea/git-scm.com,git/git-scm.com,Mokolea/git-scm.com,git/git-scm.com,git/git-scm.com,mosoft521/gitscm-next,mosoft521/gitscm-next,mosoft521/gitscm-next,mosoft521/gitscm-next,git/git-scm.com,Mokolea/git-scm.com,Mokolea/git-scm.com