fix: Remove unnecessary import of backoff.dart from sse_client_html

- Remove redundant import since Backoff is now available through public export
- Resolves dart analyzer unnecessary_import warning
- Completes the fix for exporting Backoff publicly

Co-Authored-By: [email protected] <[email protected]>

Author: devin-ai-integration[bot]

packages/event_source_client/lib/src/sse_client_html.dart

@@ -5,7 +5,6 @@ import 'dart:math' as math;
 
 import '../launchdarkly_event_source_client.dart';
 
-import 'backoff.dart';
 import 'events.dart' as ld_message_event;
 
 /// An [SSEClient] that uses the [web.EventSource] available on most browsers for web platform support.