Remove unused service principals in IAM role policy

[leoroy]

Describe the bug
Unused service principals in the IAM role policy for integ tests: https://github.com/aws/sagemaker-experiments/blob/main/tests/conftest.py#L322-L324

To Reproduce
N/A

Expected behavior
We should only keep sagemaker.amazonaws.com as the sole allowed service principal to assume the training role.

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