[iOS] Remove unused notification kAuthenticationServiceNotification.

Fixed: 464306
Change-Id: I561a1465616f16103c285620e8689b761f5a07a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2385338Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803183}

ios/chrome/app/main_controller.mm

@@ -118,11 +118,6 @@
 
 namespace {
 
-// Constants for deferring notifying the AuthenticationService of a new cold
-// start.
-NSString* const kAuthenticationServiceNotification =
-    @"AuthenticationServiceNotification";
-
 // Constants for deferring reseting the startup attempt count (to give the app
 // a little while to make sure it says alive).
 NSString* const kStartupAttemptReset = @"StartupAttempReset";