http: drop support for curl < 7.19.4

http: drop support for curl < 7.19.4

In the last commit we dropped support for curl < 7.16.0, let's
continue that and drop support for versions older than 7.19.3. This
allows us to simplify the code by getting rid of some "#ifdef"'s.

Git was broken with vanilla curl < 7.19.4 from v2.12.0 until
v2.15.0. Compiling with it was broken by using CURLPROTO_* outside any
"#ifdef" in aeae4db174 (http: create function to get curl allowed
protocols, 2016-12-14), and fixed in v2.15.0 in f18777ba6ef (http: fix
handling of missing CURLPROTO_*, 2017-08-11).

It's unclear how much anyone was impacted by that in practice, since
as noted in [1] RHEL versions using curl older than that still
compiled, because RedHat backported some features. Perhaps other
vendors did the same.

Still, it's one datapoint indicating that it wasn't in active use at
the time. That (the v2.12.0 release) was in Feb 24, 2017, with v2.15.0
on Oct 30, 2017, it's now mid-2021.

1. http://lore.kernel.org/git/c8a2716d-76ac-735c-57f9-175ca3acbcb0@jupiterrise.com;
   followed-up by f18777ba6ef (http: fix handling of missing CURLPROTO_*,
   2017-08-11)

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>