Remove unnecessary header from telemetry exporter configuration

[pleshakov]

Once this is implemented nginx/telemetry-exporter#41 and the corresponding dependency is updated in our project,
remove X-F5-OTEL from exporter configuration:
https://github.com/nginxinc/nginx-gateway-fabric/blob/0fc3902f512fb79bc53889daf2882ddfdb073081/internal/mode/static/manager.go#L550-L552

Acceptance criteria:

• Remove the header configuration
• Ensure that telemetry is successfully sent to production F5 telemetry service after the change.

[sindhushiv]

This story is blocked by the story: nginx/telemetry-exporter#41

[pleshakov]

Update: blocked by nginx/telemetry-exporter#157