Docs: Remove mention of traceToMetrics from docs

[gelicia]

Describe the bug
The getting started docs mention a feature flag here https://github.com/grafana/tempo/blob/main/docs/sources/tempo/getting-started/metrics-from-traces.md?plain=1#L49

But that feature flag was removed last year https://github.com/grafana/grafana/blob/main/pkg/services/featuremgmt/toggles-gitlog.csv#L51

Expected behavior
To update the docs to remove mention of the unnecessary feature flag.

[knylander-grafana]

work is in this PR: #4719