[engSys] Remove AZURE_POD_IDENTITY_AUTHORITY_HOST workaround

[maorleger]

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

[maorleger]

Options discussed

• B.1: Use DAC in Samples, transform AST to use test credential on run
• B.2: Use createTestCredential on run, transform to DAC as part of publishing

B.2 will be the approach I try to spike out

[github-actions[bot]]

@KarishmaGhiya @maorleger

[github-actions[bot]]

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