[iOS][MF] Clean extra reset on the observer

Change-Id: I9588c47db13e24edd46d27dacab77a4e433fe78a
Reviewed-on: https://chromium-review.googlesource.com/c/1346299Reviewed-by: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610565}

ios/chrome/browser/ui/autofill/form_input_accessory_mediator.mm

@@ -152,7 +152,6 @@
     _webState->RemoveObserver(_webStateObserverBridge.get());
     _webStateObserverBridge.reset();
     _webState = nullptr;
-    _formActivityObserverBridge.reset();
   }
   if (_webStateList) {
     _webStateList->RemoveObserver(_webStateListObserver.get());