contrib: remove vim support instructions

contrib: remove vim support instructions

The git support scripts started shipping in upstream vim in version
7.2 (2008-08-09).  Clean up contrib/ a little by removing the
instructions for people on older versions of vim.

RHEL 6 already has vim 7.2.something, so anyone on a reasonably modern
operating system should not be affected.  Users on RHEL 5 presumably
know that means sometimes missing out on niceties like syntax
highlighting, so this should be safe.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>