QA cleanup: Removed instance-disk-failure test

The test was broken, out of sync with the rest of the code, and prone to
crashes. Until someone does a better job, it's better to remove the
test. This simplifies further refactoring.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>