Remove unused NotFound raise

get_floating_ip_details() contains NotFound raise, but the exception
would never happen because previous tempest-lib's get() raises it
if HTTP404 is gotten. So this patch removes it.

Change-Id: Ia7f18dfb7491d7379e0ab55b8f84c8b10b7ebb33
diff --git a/tempest/services/compute/json/floating_ips_client.py b/tempest/services/compute/json/floating_ips_client.py
index 8e3a3c9..0354ba4 100644
--- a/tempest/services/compute/json/floating_ips_client.py
+++ b/tempest/services/compute/json/floating_ips_client.py
@@ -40,8 +40,6 @@
         url = "os-floating-ips/%s" % str(floating_ip_id)
         resp, body = self.get(url)
         body = json.loads(body)
-        if resp.status == 404:
-            raise lib_exc.NotFound(body)
         self.validate_response(schema.floating_ip, resp, body)
         return service_client.ResponseBody(resp, body['floating_ip'])