Remove ubuntu1604 from presubmit.yml

[philwo]

Ubuntu 16.04 is end-of-life, we're going to remove it from Bazel CI.

If you like you can add testing on ubuntu2004 platform which we also support.

[philwo]

Friendly ping. This config file is used to test your project on Bazel's CI: https://buildkite.com/bazel/cartographer and without the changes it will soon break.

If you are no longer interested in having your project tested on our CI, that's also fine, please just let me know.

[wohe]

@philwo I'd be happy to merge, but would need you to please sign off the commit. The easiest way is to use git commit --amend --no-edit --signoff. See https://github.com/cartographer-project/cartographer/blob/master/CONTRIBUTING.md for details. Thanks!