http: drop support for curl < 7.16.0

http: drop support for curl < 7.16.0

In the last commit we dropped support for curl < 7.11.1, let's
continue that and drop support for versions older than 7.16.0. This
allows us to get rid of some now-obsolete #ifdefs.

Choosing 7.16.0 is a somewhat arbitrary cutoff:

  1. It came out in October of 2006, almost 15 years ago.
     Besides being a nice round number, around 10 years is
     a common end-of-life support period, even for conservative
     distributions.

  2. That version introduced the curl_multi interface, which
     gives us a lot of bang for the buck in removing #ifdefs

RHEL 5 came with curl 7.15.5[1] (released in August 2006). RHEL 5's
extended life cycle program ended on 2020-11-30[1]. RHEL 6 comes with
curl 7.19.7 (released in November 2009), and RHEL 7 comes with
7.29.0 (released in February 2013).

1. http://lore.kernel.org/git/873e1f31-2a96-5b72-2f20-a5816cad1b51@jupiterrise.com

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