profiler: clear old completed list before re-using it

[paulcacheux]

What does this PR do?

Motivation

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.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: paulcacheux/free-old-profiles
Commit report: 7636238
Test service: dd-trace-go

✅ 0 Failed, 5406 Passed, 72 Skipped, 2m 55.19s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-17 20:25:25

Comparing candidate commit 2758f7e in PR branch paulcacheux/free-old-profiles with baseline commit bacf808 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 60 metrics, 2 unstable metrics.