tests: remove Dockerfile

The tests/Dockerfile was outdated (using an obsolete Ubuntu
distribution), and it's not being used by any workflow or prow job. This
Dockerfile was the only one using `scripts/install-marker.sh`, which
used a vendorized binary generated by the etcd project. This is not
needed anymore, as marker releases now provide the binaries.

Signed-off-by: Ivan Valdes <[email protected]>