AddRack: Skip multi asic & remove hack (#5254)

What is the motivation for this PR?
Test failed in unsupported platform. No more hack mode in testing.

How did you do it?
Skip it explicitly. Remove hackmode flag set to True

How did you verify/test it?
Ran the test in master 3 times successfully.
master.76122-5e1acf022

tests/configlet/test_add_rack.py

@@ -31,8 +31,9 @@ def test_add_rack(configure_dut, tbinfo, duthosts, rand_one_dut_hostname):
     global data_dir, orig_db_dir, clet_db_dir, files_dir
 
     duthost = duthosts[rand_one_dut_hostname]
+    if duthost.is_multi_asic:
+        pytest.skip('Generic patch updater does not support multiasic')
 
     log_info("sys.version={}".format(sys.version))
-    do_test_add_rack(duthost, is_storage_backend = 'backend' in tbinfo['topo']['name'],
-            hack_apply=True)
+    do_test_add_rack(duthost, is_storage_backend = 'backend' in tbinfo['topo']['name'])