Both "system_test_job" and "unit_test_job" YAML anchors contain a
"before_script" key.  TSAN job definitions first specify their own value
of the "before_script" key and then include the aforementioned YAML
anchors, which results in the value of the "before_script" key being
overridden with the value specified by the included anchor.  Given this,
remove "before_script" definitions specific to TSAN jobs as they serve
no practical purpose.

(cherry picked from commit 8ef01c7b)