Remove unused imports

lib/ui/change_collection_name_dialog.dart

@@ -1,9 +1,6 @@
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
-import 'package:photos/services/user_service.dart';
 import 'package:photos/utils/dialog_util.dart';
-import 'package:photos/utils/email_util.dart';
-import 'package:photos/utils/toast_util.dart';
 
 class ChangeCollectionNameDialog extends StatefulWidget {
   final String name;

lib/ui/thumbnail_widget.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import 'dart:math';
 import 'package:logging/logging.dart';
 import 'package:flutter/widgets.dart';
 import 'package:photos/core/cache/thumbnail_cache.dart';

lib/ui/video_widget.dart

@@ -3,7 +3,6 @@ import 'package:chewie/chewie.dart';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
-import 'package:fluttertoast/fluttertoast.dart';
 import 'package:logging/logging.dart';
 import 'package:photos/models/file.dart';
 import 'package:photos/ui/thumbnail_widget.dart';