test/nbd: do not check for /dev/nbd0

We already modprobe nbd so there is no reason to
really check for it.  There could be a slight delay
between when modprobe returns and when /dev/nbd0 is
visible, so confirm it's there just complicates things.
If somehow nbd loads but /dev/nbd0 doesn't show up, it
will obviously elsewhere where the problem was.

Plus, this was checking for /dev/nbd0 before nbd was
modprobe'd which doesn't work.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I9df91d05db95ce1135f846e96580aeb377b4b445

Reviewed-on: https://review.gerrithub.io/385715


Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>