remove opentracing doc

[trentm]

https://www.elastic.co/guide/en/apm/agent/nodejs/current/opentracing.html

This page should get removed from the docs. The OpenTracing support was deprecated back in 3.x releases when the OpenTelemetry Bridge support was added (#2641, https://www.elastic.co/guide/en/apm/agent/nodejs/current/opentelemetry-bridge.html). Since then we're on a new major: 4.x releases. Also we'd finally like to archive the https://github.com/elastic/apm-agent-nodejs-opentracing repo. It won't be getting more maintenance.