Revert "[ios] Stops observing FeedView when its VC is removed."

This reverts commit 9aa31419.

Reason for revert: Speculative revert for crash seen here:
https://chrome-isolated.appspot.com/browse?namespace=default-gzip&digest=0e2a4602cb1fa392189ea01645e1df7138fc71ea&as=StandardOutputAndStandardError-com.google.gtest.ios-chrome-eg2tests.txt

First failure: https://chrome-swarming.appspot.com/task?id=4e225dd7f44b3a10

https://ci.chromium.org/p/chrome/builders/ci/ipad-simulator/12872?blamelist=1#blamelist-tab

Original change's description:
> [ios] Stops observing FeedView when its VC is removed.
> 
> Bug: 1085419
> Change-Id: I3d2dee5f2241aa244cf1bdc2f5d7d8cf7005908d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2365095
> Reviewed-by: Ganggui Tang <gogerald@chromium.org>
> Commit-Queue: Sergio Collazos <sczs@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#799915}

TBR=gogerald@chromium.org,sczs@chromium.org

Change-Id: I93aec427308f7be16020935af14f00f1106516df
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1085419
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367637Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800237}

ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.mm

@@ -111,7 +111,6 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
 }
 
 - (void)dealloc {
-  [self.feedView removeObserver:self forKeyPath:@"contentSize"];
   [self.discoverFeedVC willMoveToParentViewController:nil];
   [self.discoverFeedVC.view removeFromSuperview];
   [self.discoverFeedVC removeFromParentViewController];
@@ -420,10 +419,8 @@ NSString* const kContentSuggestionsMostVisitedAccessibilityIdentifierPrefix =
     UIViewController* newFeedViewController = discoverFeedItem.discoverFeed;
 
     if (newFeedViewController != self.discoverFeedVC) {
-      // If previous VC is not nil, remove it from the view hierarchy and stop
-      // osberving its feedView.
+      // If previous VC is not nil, remove it from the view hierarchy.
       if (self.discoverFeedVC) {
-        [self.feedView removeObserver:self forKeyPath:@"contentSize"];
         [self.discoverFeedVC willMoveToParentViewController:nil];
         [self.discoverFeedVC.view removeFromSuperview];
         [self.discoverFeedVC removeFromParentViewController];