fast-import.c has started with a comment for nine and a half years
re-directing the reader to Documentation/git-fast-import.txt for
maintained documentation. Instead of leaving the unmaintained
documentation in place, just excise it.
Signed-off-by: Elijah Newren <newren@gmail.com> Acked-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>