contrib/install.sh: remove pull of hello-world

This removes the pull of the hello-world image from install.sh to
address privacy concerns.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #8700 from unclejack/install_no_pull

contrib/install.sh: remove pull of hello-world