http: drop support for curl < 7.19.3 and < 7.17.0 (again)

http: drop support for curl < 7.19.3 and < 7.17.0 (again)

Remove the conditional use of CURLAUTH_DIGEST_IE and
CURLOPT_USE_SSL. These two have been split from earlier simpler checks
against LIBCURL_VERSION_NUM for ease of review.

According to

  https://github.com/curl/curl/blob/master/docs/libcurl/symbols-in-versions

the CURLAUTH_DIGEST_IE flag became available in 7.19.3, and
CURLOPT_USE_SSL in 7.17.0.

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