[iOS] Remove leftBarButtonItem from ManageAccounts command.

Removes the leftBarButtonItem defined explicitly as the "Cancel" button.

Because the SettingsNavigationController is created by this command
will already have a "Done" as the right-hand bar button we avoid having
two buttons for the same purpose.

Bug: 1145592
Change-Id: I2776f7b5cdea870ebd94ee97c9dc0e6c1078fb28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529394
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Auto-Submit: Nohemi Fernandez <fernandex@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825932}

ios/chrome/browser/ui/settings/settings_navigation_controller.mm

@@ -101,7 +101,6 @@ NSString* const kSettingsDoneButtonId = @"kSettingsDoneButtonId";
       initWithRootViewController:controller
                          browser:browser
                         delegate:delegate];
-  [controller navigationItem].leftBarButtonItem = [nc cancelButton];
   return nc;
 }