chore(tornado): fix failing tornado test cases

[brettlangdon]

Fix failing tornado jobs.

When we updated the ThreadPoolExecutor implementation to pass Context instead of Span one piece of information we are not passing along is the current Span's "service".

We do not need this since spans created within the ThreadPoolExecutor will use the DD_SERVICE that is set.

These tests do not set a DD_SERVICE so it is expected that their service names are None (will inherit from their parent).

Checklist

• [x] Change(s) are motivated and described in the PR description
• [x] Testing strategy is described if automated tests are not included in the PR
• [x] Risks are described (performance impact, potential for breakage, maintainability)
• [x] Change is maintainable (easy to change, telemetry, documentation)
• [x] Library release note guidelines are followed or label changelog/no-changelog is set
• [x] Documentation is included (in-code, generated user docs, public corp docs)
• [x] Backport labels are set (if applicable)
• [x] If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• [x] Title is accurate
• [x] All changes are related to the pull request's stated goal
• [x] Description motivates each change
• [x] Avoids breaking API changes
• [x] Testing strategy adequately addresses listed risks
• [x] Change is maintainable (easy to change, telemetry, documentation)
• [x] Release note makes sense to a user of the library
• [x] Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• [x] Backport labels are set in a manner that is consistent with the release branch maintenance policy

[datadog-dd-trace-py-rkomorn[bot]]

Datadog Report

Branch report: brettlangdon/tornado.ci Commit report: ac1f5df Test service: dd-trace-py

:white_check_mark: 0 Failed, 156 Passed, 342 Skipped, 3m 26.66s Total duration (2m 5.6s time saved)