ddtrace/tracer: remove health_metrics_enabled from startup logs

[hannahkm]

What does this PR do?

Removes the health_metrics_enabled info from the startup logs.

Motivation

The health metrics are enabled by default on the Go tracer, and cannot be disabled. It is misleading to include health_metrics_enabled in the startup logs. Also, the logs were inaccurate (using runtime metrics to determine if the health metrics were "enabled").

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

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