Remove seoul.mirror.pkgbuild.com dedicated server

It failed to reboot during the last upgrade procedure. Upon logging into
the Equinix Metal console, we discovered that we lack access to all 4 of
the servers sponsored by Equinix Metal. They are under the CNCF account,
and it's not possible to transfer them to our organization.

Equinix Metal is being sunset, and the remaining 3 servers will also go
away on June 30th 2026. We can keep them until then, or until they fail
to boot like seoul.mirror.pkgbuild.com.