travis: remove the os.statvfs debug command

According to https://github.com/travis-ci/travis-ci/issues/2788 (which
was WONTFIXED), roughly half the travis-ci buildserver fleet runs off
tmpfs volumes, which have different notions of space-free and space-used
than real filesystems. We've adapted our tests to avoid relying upon
space-used>0, and no longer need this call to investigate the problem.

refs ticket:2290