Avoid loading virtualbox driver on Jenkins slaves

Trying to destroy a VM before the machine is available loads the
virtualbox driver, VirtualBox isn't installed on our Jenkins slaves

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Delete unused rake task

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>