[chore] Remove deprecated documentation (open-telemetry#11483)

Ex. Adding a feature - Explain what this achieves.-->
#### Description

Removes pages that were moved to opentelemetry.io. These pages now live
at:

-
https://opentelemetry.io/docs/collector/internal-telemetry/#use-internal-telemetry-to-monitor-the-collector
- https://opentelemetry.io/docs/collector/troubleshooting/

Neither of the md docs in opentelemetry-collector are among the top 10
most visited pages according to Github traffic stats (we don't have
visibility into individual pages, only the top 10)