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

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>