[iOS] UnifiedConsentCoordinator needs to be released after being dismissed

|_unifiedConsentCoordinator| should be removed once the sign-in is done
and the user consent is recorded.
This is to avoid to receive notifications related to the UIScrollView
in UnifiedConsentViewController, like:
-[ChromeSigninViewController unifiedConsentCoordinatorDidReachBottom:]

Bug: 918557
Change-Id: I9d57dbf4d9f37f636fa44105f268d7089e192abf
Reviewed-on: https://chromium-review.googlesource.com/c/1393372Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619615}

ios/chrome/browser/ui/authentication/chrome_signin_view_controller.mm

@@ -289,6 +289,8 @@ enum AuthenticationState {
     _didFinishSignIn = YES;
     [_delegate didAcceptSignIn:self showAccountsSettings:showAccountsSettings];
   }
+  _unifiedConsentCoordinator.delegate = nil;
+  _unifiedConsentCoordinator = nil;
 }
 
 - (void)acceptSignInAndCommitSyncChanges {