We use this workaround to bypass Managed Identity in the DAC chain when running samples.

Removing this workaround requires using createTestCredential instead of DefaultAzureCredntial in sample runs.

AC:

• createTestCredential is used for running samples-dev
• DefaultAzureCredential continues to be the credential used in published samples
• All sample CI builds continue to work
• the env var is removed from https://github.com/Azure/azure-sdk-for-js/blob/main/eng/pipelines/templates/jobs/live.tests.yml#L279 and https://github.com/Azure/azure-sdk-for-js/blob/main/eng/pipelines/templates/jobs/live.tests.yml#L315