drop book.rake
drop book.rake

We don't actually run the remote_genbook task anymore (we do
remote_genbook2).  This isn't hurting anything, but it makes
it harder to find the actual tasks of interest.

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