deprecation: Remove stale S3DownloadMode from test_session.py

[navinsoni]

Issue #, if available:

Description of changes:
This PR is to remove stale S3DownloadMode field present in S3Input.
This field was removed from code in this PR

Testing done:

===================================================================================== 4999 passed, 447 skipped, 1 xfailed, 1 xpassed, 3046 warnings in 338.24s (0:05:38) =====================================================================================
py38 run-test: commands[1] | coverage report -i --fail-under=86

---

TOTAL 14751 1518 90%
__________________________________________________________________________________________________________________________ summary ___________________________________________________________________________________________________________________________
black-format: commands succeeded
flake8: commands succeeded
pylint: commands succeeded
docstyle: commands succeeded
sphinx: commands succeeded
doc8: commands succeeded
twine: commands succeeded
ERROR: py36: InterpreterNotFound: python3.6
ERROR: py37: InterpreterNotFound: python3.7
py38: commands succeeded

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• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have passed the region in to all S3 and STS clients that I've initialized as part of this change.
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)
• I have used unique_name_from_base to create resource names in integ tests (if appropriate)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: f77ed77
• Result: SUCCEEDED
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[ahsan-z-khan]

LGTM!

[shreyapandit]

Thanks for taking this on!