Remove TEST_LICENSE_MODE env var

What does this MR do and why?

Setting GITLAB_LICENSE_MODE to test performs essentially the same function as setting TEST_LICENSE_MODE to true, so we don't need both variables. Since GITLAB_LICENSE_MODE is used elsewhere this removes TEST_LICENSE_MODE.

The test license mode seems to be only relevant to CustomersDot testing so it shouldn't affect other pipelines.

Part of gitlab-org/quality/quality-engineering/team-tasks#1630 (closed)
Depends on gitlab-qa!1125 (merged)

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