Documentation/githooks: post-rewrite-copy-notes never existed

The documentation for the post-rewrite hook contains a paragraph from its early development, where the automatic notes copying facilities were not part of the series and thus this had to be a hook. Later versions of the series implemented notes copying as a core feature. Thus mentioning post-rewrite-copy-notes was never correct. As the other hooks do not have a "there is no default hook, but..." sentence unless they ship a sample hook in either templates or contrib, we simply remove the whole paragraph. Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>