storage: Resolve Coverity UNUSED_VALUE

Since cd4d547576

Coverity notes that setting 'ret = -3' prior to the unconditional
setting of 'ret = 0' will cause the value to be UNUSED.

Since the comment indicates that it is expect to allow the code
to continue, just remove the ret = -3 setting.

Signed-off-by: John Ferlan <jferlan@redhat.com>

src/storage/storage_backend_fs.c

@@ -128,7 +128,6 @@ virStorageBackendProbeTarget(virStorageSourcePtr target,
                 virReportError(VIR_ERR_INTERNAL_ERROR,
                                _("cannot probe backing volume format: %s"),
                                target->backingStore->path);
-                ret = -3;
             } else {
                 target->backingStore->format = rc;
             }