chore(tracer): unpin system-tests (#6832)

Partial revert of #6780
As 1.x version problem was fixed in
#6826, we shouldn't need to
pin system tests version for the trunk branch anymore.

## 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] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly 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](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

.github/workflows/system-tests.yml

@@ -55,7 +55,6 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: 'DataDog/system-tests'
-          ref: '84a72c386a095dcbb785f9be11e61814ead61345'
 
       - name: Checkout dd-trace-py
         if: needs.needs-run.outputs.outcome == 'success'
@@ -165,7 +164,6 @@ jobs:
         uses: actions/checkout@v3
         with:
           repository: 'DataDog/system-tests'
-          ref: '84a72c386a095dcbb785f9be11e61814ead61345'
 
       - uses: actions/setup-python@v4
         if: needs.needs-run.outputs.outcome == 'success'