Remove example dockerfile

Fixes #6474
Fixes #6191

- removed the outdated and un-maintained example dockerfile from the repo.
- since the dockerfile is not maintained, and `play-with-docker` no longer
works without actually signing in, the dockerfile just caused issues and
users and adopters should instead be re-directed to `theia-apps` in order
to view concrete and thorough examples of docker images.

Signed-off-by: vince-fugnitto <vincent.fugnitto@ericsson.com>