autocomplete [nfc]: Cut "candidate" type param from AutocompleteView

This is no longer used since cfce389 introduced computeResults and
eliminated getSortedItemsToTest and testItem from the base class.

Author: gnprice

lib/model/autocomplete.dart

@@ -187,7 +187,7 @@ class AutocompleteViewManager {
 ///  * On reassemble, call [reassemble].
 ///  * When the object will no longer be used, call [dispose] to free
 ///    resources on the [PerAccountStore].
-abstract class AutocompleteView<QueryT extends AutocompleteQuery, ResultT extends AutocompleteResult, CandidateT> extends ChangeNotifier {
+abstract class AutocompleteView<QueryT extends AutocompleteQuery, ResultT extends AutocompleteResult> extends ChangeNotifier {
   AutocompleteView({required this.store});
 
   final PerAccountStore store;
@@ -284,7 +284,7 @@ abstract class AutocompleteView<QueryT extends AutocompleteQuery, ResultT extend
   }
 }
 
-class MentionAutocompleteView extends AutocompleteView<MentionAutocompleteQuery, MentionAutocompleteResult, User> {
+class MentionAutocompleteView extends AutocompleteView<MentionAutocompleteQuery, MentionAutocompleteResult> {
   MentionAutocompleteView._({
     required super.store,
     required this.narrow,
@@ -589,7 +589,7 @@ class UserMentionAutocompleteResult extends MentionAutocompleteResult {
 
 // TODO(#234): // class WildcardMentionAutocompleteResult extends MentionAutocompleteResult {
 
-class TopicAutocompleteView extends AutocompleteView<TopicAutocompleteQuery, TopicAutocompleteResult, String> {
+class TopicAutocompleteView extends AutocompleteView<TopicAutocompleteQuery, TopicAutocompleteResult> {
   TopicAutocompleteView._({required super.store, required this.streamId});
 
   factory TopicAutocompleteView.init({required PerAccountStore store, required int streamId}) {

lib/widgets/autocomplete.dart

@@ -24,14 +24,14 @@ abstract class AutocompleteField<QueryT extends AutocompleteQuery, ResultT exten
 
   Widget buildItem(BuildContext context, int index, ResultT option);
 
-  AutocompleteView<QueryT, ResultT, CandidateT> initViewModel(BuildContext context);
+  AutocompleteView<QueryT, ResultT> initViewModel(BuildContext context);
 
   @override
   State<AutocompleteField<QueryT, ResultT, CandidateT>> createState() => _AutocompleteFieldState<QueryT, ResultT, CandidateT>();
 }
 
 class _AutocompleteFieldState<QueryT extends AutocompleteQuery, ResultT extends AutocompleteResult, CandidateT> extends State<AutocompleteField<QueryT, ResultT, CandidateT>> with PerAccountStoreAwareStateMixin<AutocompleteField<QueryT, ResultT, CandidateT>> {
-  AutocompleteView<QueryT, ResultT, CandidateT>? _viewModel;
+  AutocompleteView<QueryT, ResultT>? _viewModel;
 
   void _initViewModel() {
     _viewModel = widget.initViewModel(context)