Removes auto-updater from workstation

[kushaldas]

This PR removes the securedrop autoupdater.

We can get it back in future, for now use the Qubes
updater UI.

How to test?

In dom0 after getting this branch.

make prep-dom0

[redshiftzero]

I realize that you were asked you to do this by another team member but I am blocking merge on this into master due to the security implications, see #238 (securedrop-workstation based VMs will never report updates using the qubes updater)

[eloquence]

We discussed this further today. Even though we're not formally in production yet, it's our policy to not introduce security regressions during development, and the existing cron-job, in spite of its limitations, provides a core security feature right now. The current recommendation to developers who find it disruptive is to temporarily delete the symlink in /etc/cron.daily (it will be reinstated on the next make all run).

We've also agreed to improve the updater story ASAP; see discussion in freedomofpress/securedrop-updater#34. In the meantime, this PR can be closed.

[redshiftzero]

closing as discussed