lxc-download: Rely on HTTPS only

lxc-download: Rely on HTTPS only

GPG has been a major source of issues over the years with various
attacks on the key network as well as client side issues making it hard
to retrieve our keys.

Back when we introduced the image server, SSL certificates were still
expensive and annoying to setup, so not something we'd have expected
potential mirrors to setup for us. They were also issued for multiple
years, making a compromise of such a certificate quite problematic.

But things have changed since, we now have completely free, very easily
deployable SSL certificates everywhere with the majority of those being
shortlived and with good reporting of issued certificates.

With that, we can now deprecate the GPG validation, disable the fallback
to non-HTTPS download and rely on our indices being accurate because
they've been downloaded from a server with a valid certificate.

This puts LXC more in line with what LXD has done since the beginning
and should offer a more reliable user experience.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>