SigninManagerBase::Observer does not need friends

After https://crrev.com/c/1344089, SigninManagerBase::Observer doesn't
have any private methods, so SigninManager no longer needs to be a
friend.

Change-Id: I87ddfb4c25657476bd00f734a8d22c07f5b05a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642047Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#665886}

components/signin/core/browser/signin_manager_base.h

@@ -65,10 +65,6 @@ class SigninManagerBase {
 
    protected:
     virtual ~Observer() {}
-
-   private:
-    // SigninManagers that fire notifications.
-    friend class SigninManager;
   };
 
 // On non-ChromeOS platforms, SigninManagerBase should only be instantiated