rebuilderd_worker: unset {BOOTSTRAP,HOST}MIRROR

Upstream archlinux-repro was already using europe.mirror.pkgbuild.com as
its bootstrap and regular mirror. Furthermore, since [1] it has switched
to the Geo mirror. Remove both vars from /etc/archlinux-repro/repro.conf
and use the default mirror values (which are more than suitable).

[1] https://github.com/archlinux/archlinux-repro/commit/c024b892d07a

roles/rebuilderd_worker/files/repro.conf

 NOCHECK=1
 CACHEDIR=/var/lib/rebuilderd-worker/cache
-BOOTSTRAPMIRROR="https://europe.mirror.pkgbuild.com/iso/latest"
-HOSTMIRROR="https://europe.mirror.pkgbuild.com/\$repo/os/\$arch"