Removing flaky test

[satween]

What does this PR do?

Removes JointToStringVsStringBuilderPerformanceTest from dd-sdk-android-core.

Motivation

This test was created in order to verify that in most cases buildString is faster than joinToString and was needed only at development stage. The test is flaky because of uncontrolled initial conditions, it's not guard against changes and basically only verifies the approach correctness.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[datadog-prod-us1-6]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 71.84% (+0.05%)

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