[route_check] remove check-frr_patch mock (sonic-net#2732)

The test fails with python3.7 (works in 3.9) when stopping patch which hasn't been started. We can always mock check_output call and if FRR_ROUTES is not defined return empty dictionary by the mock.

#### What I did

Removed check_frr_patch mock to fix UT running on python3.7

#### How I did it

Removed the mock

#### How to verify it

Run unit test in stretch env

tests/route_check_test.py

@@ -277,17 +277,12 @@ def test_route_check(self, mock_dbs, test_num):
         with patch('sys.argv', ct_data[ARGS].split()), \
             patch('route_check.subprocess.check_output') as mock_check_output:
 
-            check_frr_patch = patch('route_check.check_frr_pending_routes', lambda: [])
+            routes = ct_data.get(FRR_ROUTES, {})
 
-            if FRR_ROUTES in ct_data:
-                routes = ct_data[FRR_ROUTES]
+            def side_effect(*args, **kwargs):
+                return json.dumps(routes)
 
-                def side_effect(*args, **kwargs):
-                    return json.dumps(routes)
-
-                mock_check_output.side_effect = side_effect
-            else:
-                check_frr_patch.start()
+            mock_check_output.side_effect = side_effect
 
             ret, res = route_check.main()
             expect_ret = ct_data[RET] if RET in ct_data else 0
@@ -299,8 +294,6 @@ def side_effect(*args, **kwargs):
             assert ret == expect_ret
             assert res == expect_res
 
-            check_frr_patch.stop()
-
     def test_timeout(self, mock_dbs, force_hang):
         # Test timeout
         ex_raised = False