send-email: remove non-working support for "sendemail.smtpssl"

send-email: remove non-working support for "sendemail.smtpssl"

Remove the already dead code to support "sendemail.smtpssl" by finally
removing the dead code supporting the configuration option.

In f6bebd121ac (git-send-email: add support for TLS via
Net::SMTP::SSL, 2008-06-25) the --smtp-ssl command-line option was
documented as deprecated, later in 65180c66186 (List send-email config
options in config.txt., 2009-07-22) the "sendemail.smtpssl"
configuration option was also documented as such.

Then in in 3ff15040e22 (send-email: fix regression in
sendemail.identity parsing, 2019-05-17) I unintentionally removed
support for it by introducing a bug in read_config().

As can be seen from the diff context we've already returned unless
$enc i defined, so it's not possible for us to reach the "elsif"
branch here. This code was therefore already dead since Git v2.23.0.

So let's just remove it. We were already 11 years into a stated
deprecation period of this variable when 3ff15040e22 landed, now it's
around 13. Since it hasn't worked anyway for around 2 years it looks
like we can safely remove it.

The --smtp-ssl option is still deprecated, if someone cares they can
follow-up and remove that too, but unlike the config option that one
could still be in use in the wild. I'm just removing this code that's
provably unused already.

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