Fix linting errors

Author: mugikhan

example/custom_functions_example.dart

@@ -1,4 +1,3 @@
-import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 

example/linux_cli_example.dart

@@ -1,4 +1,3 @@
-import 'dart:async';
 import 'dart:ffi';
 
 import 'package:sqlite3/common.dart';

lib/src/native/database/native_sqlite_connection_impl.dart

@@ -264,7 +264,7 @@ void _sqliteConnectionIsolate(_SqliteConnectionParams params) async {
     await client.post(const InitDb());
   }
 
-  final db = await params.openFactory.open(SqliteOpenOptions(
+  final db = params.openFactory.open(SqliteOpenOptions(
       primaryConnection: params.primary, readOnly: params.readOnly));
 
   runZonedGuarded(() async {