Remove unallowed ports from policy test

PolicyPrefsTest.PolicyToPrefsMapping is currently disabled due to
flakiness. One sub-test (see policy_test_cases.json) used the
ExplicitlyAllowedNetworkPorts with a range of values, which are no
longer valid (see https://crrev.com/c/3655041). This would cause the
test to fail when re-enabling that test. Since no more values are
actually valid for that policy, we will simply check the default case
and the policy set to an empty list. The policy will remain in place for
potential future use of the same functionality.

BUG=1199642,1220079,1210779,1196846

Change-Id: I6f517a1ae2518f6088c7bd2faa3f221ae8b4b7ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3714129
Auto-Submit: Alexander Hendrich <hendrich@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1016629}

chrome/test/data/policy/policy_test_cases.json

@@ -16911,18 +16911,23 @@
     ],
     "policy_pref_mapping_tests": [
       {
+        "note": "default value",
+        "policies": {},
+        "prefs": {
+          "net.explicitly_allowed_network_ports": {
+            "default_value": [],
+            "location": "local_state"
+          }
+        }
+      },
+      {
+        "note": "no ports are currently allowed to be blocked, but policy remains in place for potential future use",
         "policies": {
-          "ExplicitlyAllowedNetworkPorts": [
-            "554",
-            "10080"
-          ]
+          "ExplicitlyAllowedNetworkPorts": []
         },
         "prefs": {
           "net.explicitly_allowed_network_ports": {
-            "value": [
-              554,
-              10080
-            ],
+            "value": [],
             "location": "local_state"
           }
         }