auto/build: drop checks for conditions that are satisfied in supported build environments.
- isohybrid is available in our Vagrant build VMs. The manual build documentation is totally outdated and broken, nobody complained about it, which suggests nobody tried to follow it since 1+ years. And anyway, if isohybrid is not installed, the build will fail loudly later on. - $LB_BINARY_IMAGES is correctly set because auto/config passes "--binary-images iso" to "lb config". Checking it here brings no value and just makes things harder for anyone who would want to add support for other kinds of images. - It's been a while since we've stopped releasing source tarballs :) One can't reach auto/config via "rake build" if one is not building from Git, and building without rake is unsupported (good luck with that).