Drop Installer's dependency on isomd5sum

Only its checkisomd5 utility is “used” in the Installer’s code, and we don’t embed checksum info in the ISO, so it brings no value. isomd5sum is not in Ubuntu/main, so it makes it harder than needed to install Tails Installer on Ubuntu.

This work should be done in the feature/jessie (and debian/sid) branches only, no need to bother with the Wheezy version.

Parent Task: #8549 (closed)

Original created by @intrigeri on 10374 (Redmine)

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