autocomplete: Remove the refreshStaleUserResults method

This method was used to search for the new user results in autocomplete
when the users in store were modified during an in-progress computation.
Along with this, `_startSearch` would do the same thing by retrying,
resulting in duplicate actions. By removing this, the unnecessary search
for user results is prevented.

Besides, this commit also removes some live-updating behavior of
user-mention autocomplete results in order to provide a better UX.
(To read more about this decision, check out
zulip#693 (review))

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -125,23 +125,11 @@ class AutocompleteViewManager {
     assert(removed);
   }
 
-  void handleRealmUserAddEvent(RealmUserAddEvent event) {
-    for (final view in _mentionAutocompleteViews) {
-      view.refreshStaleUserResults();
-    }
-  }
-
   void handleRealmUserRemoveEvent(RealmUserRemoveEvent event) {
-    for (final view in _mentionAutocompleteViews) {
-      view.refreshStaleUserResults();
-    }
     autocompleteDataCache.invalidateUser(event.userId);
   }
 
   void handleRealmUserUpdateEvent(RealmUserUpdateEvent event) {
-    for (final view in _mentionAutocompleteViews) {
-      view.refreshStaleUserResults();
-    }
     autocompleteDataCache.invalidateUser(event.userId);
   }
 
@@ -208,15 +196,6 @@ class MentionAutocompleteView extends ChangeNotifier {
     }
   }
 
-  /// Recompute user results for the current query, if any.
-  ///
-  /// Called in particular when we get a [RealmUserEvent].
-  void refreshStaleUserResults() {
-    if (_query != null) {
-      _startSearch(_query!);
-    }
-  }
-
   /// Called when the app is reassembled during debugging, e.g. for hot reload.
   ///
   /// This will redo the search from scratch for the current query, if any.

lib/model/store.dart

@@ -413,7 +413,6 @@ class PerAccountStore extends ChangeNotifier with StreamStore {
     } else if (event is RealmUserAddEvent) {
       assert(debugLog("server event: realm_user/add"));
       users[event.person.userId] = event.person;
-      autocompleteViewManager.handleRealmUserAddEvent(event);
       notifyListeners();
     } else if (event is RealmUserRemoveEvent) {
       assert(debugLog("server event: realm_user/remove"));