[Features Platform] Remove TEMPORAL_FEATURES_DISABLE_WORKFLOW_COMPLETION_CHECK workaround

[dandavison]

In #577 the env var TEMPORAL_FEATURES_DISABLE_WORKFLOW_COMPLETION_CHECK was introduced to workaround a problem when running tests against cloud. This should be resolved correctly and the env var removed.

The problem is described here

We have a bit of older logic that checks after our tests if any workflows are still running. Repeatedly every 100ms, to a max of 5s, it makes a Go SDK ListOpenWorkflowExecutions + ListClosedWorkflowExecutions call to see if any of the returned workflows have a status of WORKFLOW_EXECUTION_STATUS_RUNNING. If after 5s there is still one with that status, we fail. We saw sometime approx between Oct 10th and Oct 25th, our assertion here started failing in cloud (and still does). The workflows were completing (in milliseconds) but even after 5s these were returning stale data in cloud saying that status was still running.

#579 has updated to use the preferred ListWorkflow API, but the problem remains.