[Extensions] Update PolicyTests to manifest v2

PolicyTests have been updated to manifest v2.
Remove the allowance for manifest v1 extensions.

Bug: 816679
Change-Id: Ifc1db03aa42db2ae367389d4ec3df4efb18573db
Reviewed-on: https://chromium-review.googlesource.com/1026073Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553549}

chrome/browser/policy/policy_browsertest.cc

@@ -740,13 +740,6 @@ class PolicyTest : public InProcessBrowserTest {
     }
   }
 
-  void SetUpCommandLine(base::CommandLine* command_line) override {
-    InProcessBrowserTest::SetUpCommandLine(command_line);
-    // TODO(devlin): Remove this. See https://crbug.com/816679.
-    command_line->AppendSwitch(
-        extensions::switches::kAllowLegacyExtensionManifests);
-  }
-
   void SetScreenshotPolicy(bool enabled) {
     PolicyMap policies;
     policies.Set(key::kDisableScreenshots, POLICY_LEVEL_MANDATORY,