satlab: remove obsolete openssh_server container

The openssh_server container was added in 2021 (b/202024560) to act as
an SSH reverse-tunnel proxy for Autotest AU tests using TLS
CreateFakeOmaha(exposed_via_proxy=True).

All tests using FakeOmaha were migrated or removed by Feb 2023, and the
autotest_internal_rsa private key required to authenticate to
openssh_server was removed from the drone image in Apr 2023
(b/278610907). Remove the unused container from docker-compose, startup
scripts, and Cloud Build.

BUG=b:580874220
TEST=manual

Change-Id: I42569fa51d0d2fbeee425c22ea227c2f7208726a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/satlab/+/8706052
Tested-by: Tomasz Jakubowski <jakubowskit@google.com>
Commit-Queue: Tomasz Jakubowski <jakubowskit@google.com>
Reviewed-by: Rafał Gaweł <rafalgawel@google.com>