Metrics: Delete UI for Grafana authentication

⚠ Removal should take place in %16.0 or later.

As part of #346541 (closed) remove Grafana authentication section in Settings > Monitor

Relevant files

frontend

• app/assets/javascripts/grafana_integration
• spec/frontend/grafana_integration

documentation

• doc/operations/metrics/embed_grafana.md

Implementation plan

1. delete Grafana authentication section in Settings > Monitor
2. delete relevant docs

Verification steps

These areas of the app should remain unavailable & without regressions to other features: