http: drop support for curl < 7.18.0 (again)

http: drop support for curl < 7.18.0 (again)

In 644de29e220 (http: drop support for curl < 7.19.4, 2021-07-30) we
dropped support for curl < 7.19.4, so we can drop support for this
non-obvious dependency on curl < 7.18.0.

It's non-obvious because in curl's hex version notation 0x071800 is
version 7.24.0, *not* 7.18.0, so at a glance this patch looks
incorrect.

But it's correct, because the existing version check being removed
here is wrong. The check guards use of the following curl defines:

    CURLPROXY_SOCKS4                7.10
    CURLPROXY_SOCKS4A               7.18.0
    CURLPROXY_SOCKS5                7.10
    CURLPROXY_SOCKS5_HOSTNAME       7.18.0

I.e. the oldest version that has these is in fact 7.18.0, not
7.24.0. That we were checking 7.24.0 is just an mistake in
6d7afe07f29 (remote-http(s): support SOCKS proxies, 2015-10-26),
i.e. its author confusing base 10 and base 16.

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