Dockerfile bugfix for Docker Trusted Builds

For some reason the recent ubuntu trusted builds are consistently failing on this line.  I don't think there's any harm in removing this line from the Dockerfile.

https://index.docker.io/u/numenta/nupic-ubuntu/builds_history/14933/
and https://index.docker.io/u/numenta/nupic/builds_history/15031/

cc: @allanino