chore: remove stale Jaeger agent scrape rule

[DaedalusG]

Problem

Jaeger deprecated and removed the standalone jaeger-agent in favor of the OpenTelemetry Collector: jaegertracing/jaeger#4739

Sourcegraph's active Helm deployment uses the maintained sourcegraph/jaeger-all-in-one image with Jaeger v2 configuration and OTLP ingestion. However, the Prometheus configuration still contained a relabel rule from 2021 that dropped scrape targets whose container name was jaeger-agent.

The rule did not deploy or reference the retired image, but it became dead configuration once the agent workload was removed.

Changes

• remove the stale jaeger-agent Prometheus scrape-drop rule

This does not change the Jaeger workload, image, ports, storage, or tracing configuration.

Related work

• Sourcegraph package and image cleanup: https://github.com/sourcegraph/sourcegraph/pull/14456
• Merged infrastructure registry and scanning cleanup: https://github.com/sourcegraph/infrastructure/pull/7458

Test plan

• helm lint charts/sourcegraph
• git diff --check
• verify no jaeger-agent references remain under charts/