Remove ProcessRunner code leaving Nox to run Procfiles

[matthewmcgarvey]

Closes #714
Closes #688

The previous PR added Nox as a process runner. Since it's a shard instead of a process installed on users' machines, it's guaranteed to be present so we don't need the ProcessRunner code anymore. Consequently also removed the use of the ensure_process_runner_installed task which I never even knew existed.

This is a breaking change. Projects will need to update script/system_check and delete these lines

# Only if this isn't CI
if [ -z "$CI" ]; then
  lucky ensure_process_runner_installed
fi

[jwoertink]

Sweet!