Remove unnecessary "is not None" check

From a review comment on https://review.openstack.org/30051 remove
the "is not None" check, as the assignment in the try block will
never assign None as value as the int() built-in will not return it.

There was some concern of long-term maintenance of the DiskFile
class's _quarantine method raising exceptions. If that routine were
ever mistakenly changed to NOT raise an exception, subtle problems
could creep into the code (see https://review.openstack.org/53237).
We address this concern by raising an exception explicitly at the
call sites of DiskFile._quarantine().

Change-Id: I1729a2d77a6b72b4494b24a8838b47ad5272c075
Signed-off-by: Peter Portante <peter.portante@redhat.com>