Remove unnecessary reset for identity::IdentityManagerObserverBridge

As discussed in the review for CL1326151, there is no need to do
this explicitly on dealloc() since the std::unique_ptr for this
object will be destroyed as part of the deallocation. Thus, we're
removing the call to .reset() to prevent further confusion.

Bug: 903262
Change-Id: Iec51c701501de948d32ce557ca37a7850db7b107
Reviewed-on: https://chromium-review.googlesource.com/c/1337611Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#611223}

ios/chrome/browser/ui/bookmarks/bookmark_promo_controller.mm

@@ -68,8 +68,6 @@
 
 - (void)dealloc {
   [_signinPromoViewMediator signinPromoViewRemoved];
-  if (!_isIncognito)
-    _identityManagerObserverBridge.reset();
 }
 
 - (void)hidePromoCell {