[iOS] Fixing DCHECK in UserSigninViewController

The activity indicator should not be added in -[UserSigninViewController
viewDidLoad].
The indicator is added when it is started, by -[UserSigninViewController
startAnimatingActivityIndicator].

Bug introduced with: crrev.com/c/2079111

Bug: 971989
Change-Id: I9f8d8ee37399a1fba0197112f1998cdb7bfa5aec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087214
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746764}

ios/chrome/browser/ui/authentication/signin/user_signin/user_signin_view_controller.mm

@@ -133,7 +133,6 @@ enum AuthenticationButtonType {
 
   [self addConfirmationButtonToView];
   [self embedUserConsentView];
-  [self addActivityIndicatorToView];
   [self addSkipSigninButtonToView];
 
   [self.view addSubview:self.gradientView];