webadmin: Remove redundant super.validate() from NewDiskModel

NewDiskModel validate() contained 2 calls to the super.validate() when
result of the first call is completely ignored.

Change-Id: Id744a08dd74d1536218363f3d9f19cd13cc4c97b
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>