docs(tracing): remove Helicone — gateway only, no tracing interface support

Per reviewer feedback from @seratch: Helicone integrates with the
Responses API but does not implement the Agents SDK tracing interface.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: m1lestones

docs/tracing.md

@@ -221,4 +221,3 @@ The following community and vendor integrations support the OpenAI Agents SDK tr
 -   [Traccia](https://traccia.ai/docs/integrations/openai-agents)
 -   [PromptLayer](https://docs.promptlayer.com/languages/integrations#openai-agents-sdk)
 -   [HoneyHive](https://docs.honeyhive.ai/v2/integrations/openai-agents)
--   [Helicone](https://docs.helicone.ai/gateway/integrations/openai-agents)