Bug#410878: d-i uses mirror/http/proxy setting when it (probably) shouldn't

[Max Truxa <me@maxtruxa.com>]

This just bit me on Debian 10, although it seems to affect only
scripts run via in-target (i.e. `wget ...` works as expected,
`in-target wget ...` doesn't).

Over the years the relevant lines have been changed slightly and moved
to /lib/chroot-setup.sh:
https://salsa.debian.org/installer-team/debian-installer-utils/-/blob/master/chroot-setup.sh#L136

        RET=$(debconf-get mirror/protocol || true)
        if [ "$RET" = "http" ]; then
                RET=$(debconf-get mirror/http/proxy || true)
                if [ "$RET" ]; then
                        http_proxy="$RET"
                        export http_proxy
                fi
        fi

As far as I can tell this is unnecessary for "in-target" APT to work,
as proxy settings are written to /target/etc/apt/apt.conf already. And
as others have pointed out, HTTP requests issued by any other program
should not be affected by "mirror/http/proxy".