update sagemaker agent setup doc as secrets aren't required anymore

[samhita-alla]

Tracking issue

https://github.com/flyteorg/flyte/issues/

Why are the changes needed?

What changes were proposed in this pull request?

How was this patch tested?

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.08%. Comparing base (8bb5b8c) to head (60e018f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5138      +/-   ##
==========================================
- Coverage   59.09%   59.08%   -0.02%     
==========================================
  Files         645      645              
  Lines       55540    55540              
==========================================
- Hits        32824    32818       -6     
- Misses      20127    20133       +6     
  Partials     2589     2589              

Flag	Coverage Δ
unittests	59.08% <ø> (-0.02%)	⬇️

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