[9.18] fix: ci: No need to delete the "only" keyword in generate-tsan-stress-jobs.py

29fd756408 replaced "only" with "rules" in
.gitlab-ci.yml but forgot to drop the removal from here, hence the
script was broken.

(cherry picked from commit 6e2272d769)

Backport of MR !10185

Merge branch 'mnowak/do-not-delete-only-keyword-in-generate-tsan-stress-jobs' into 'bind-9.18'

See merge request isc-projects/bind9!10188

util/generate-tsan-stress-jobs.py

@@ -29,7 +29,6 @@ for tsan_job in "gcc:tsan", "clang:tsan":
         tsan_stress_test_job["needs"] = [
             {"pipeline": "$PARENT_PIPELINE_ID", "job": tsan_job}
         ]
-        del tsan_stress_test_job["only"]
 
         print(
             yaml.dump(