http: drop support for curl < 7.11.1

http: drop support for curl < 7.11.1

Drop support for this ancient version of curl and simplify the code by
allowing us get rid of some "#ifdef"'s.

Git will not build with vanilla curl older than 7.11.1 due our use of
CURLOPT_POSTFIELDSIZE in 37ee680d9b
(http.postbuffer: allow full range of ssize_t values,
2017-04-11). This field was introduced in curl 7.11.1.

We could solve these compilation problems with more #ifdefs,
but it's not worth the trouble. Version 7.11.1 came out in
March of 2004, over 17 years ago. Let's declare that too old
and drop any existing ifdefs that go further back. One
obvious benefit is that we'll have fewer conditional bits
cluttering the code.

This patch drops all #ifdefs that reference older versions
(note that curl's preprocessor macros are in hex, so we're
looking for 070b01, not 071101).

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>