webadmin: remove redundant null check in importFileSD

importFileStorageAddDomain -> 'sdToAdd' variable can't be null;
it's verified in the calling method (importFileStorageConnect).
Hence, removing null check since findbugs suspect it as a possible NPE
(null shouldn't be passed to StorageDomainManagementParameter ctr).

Change-Id: I62eaff9b2ebffccb938f9db646548d61c4169af8
Signed-off-by: Daniel Erez <derez@redhat.com>