Langfuse: Remove warning "Creating a new trace without a parent span is not recommended ..."

[sjrl]

The warning message isn't helpful since the user can't take any action to "fix" anything.

Also often times the message isn't valid, there isn't a requirement that a parent span has to be started from any particular operation. In this case it popped up again when running the Agent in isolation with the warning message "Creating a new trace without a parent span is not recommended for operation 'haystack.agent.run'." which actually is fine.