hash-object doc: stop mentioning git-cvsimport

hash-object doc: stop mentioning git-cvsimport

Remove a reference to git-cvsimport in the intro. As can be seen from
the history of this command[1] it was originally intended for use with
git-cvsimport, but how it uses it (and that it uses it at all) is
irrelevant trivia at this point.

1. See 7672db20c2 ("[PATCH] Expose object ID computation functions.",
   2005-07-08) and 8b8840e046 ("[PATCH] cvsgit fixes: spaces in
   filenames and CVS server dialog woes", 2005-08-15).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>