chore (internal/civisibility): remove the unused impacted test backend call

[tonyredondo]

What does this PR do?

This PR removes the backend call to the impacted tests endpoint.

Motivation

We decided that all the calculations are going to be made on the tracer, so we don't need the backend fallback anymore.
Documented here: https://docs.google.com/document/d/13EBqv-8XKGAmDR4J10Lwu1UQYXGfYUwjmOBDQaKKqbw

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.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• 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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-05-29 12:24:31

Comparing candidate commit 04d7438 in PR branch tony/civisibility-remove-impacted-tests-backend-call with baseline commit 53d2e6e in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 21 metrics, 0 unstable metrics.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: tony/civisibility-remove-impacted-tests-backend-call
Commit report: 6d79bdb
Test service: dd-trace-go

✅ 0 Failed, 4523 Passed, 64 Skipped, 3m 32.66s Total Time

[anmarchenko]

this looks already good, the changes will be not so big to work with remote branches only