[Extensions] Update ExternalInstallErrorTests to manifest v2

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

Bug: 816679
Change-Id: I59bcbcaf0d1d15e6270e485398091f3e714f217b
Reviewed-on: https://chromium-review.googlesource.com/1024609Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553394}

chrome/browser/extensions/external_install_error_browsertest.cc

@@ -22,8 +22,6 @@ class ExternalInstallErrorTest : public ExtensionBrowserTest {
   ExternalInstallErrorTest() = default;
   ~ExternalInstallErrorTest() override = default;
 
-  bool ShouldAllowLegacyExtensionManifests() override { return true; }
-
  private:
   DISALLOW_COPY_AND_ASSIGN(ExternalInstallErrorTest);
 };