Cleanup unused flyteadmin config

[Sovietaced]

Why are the changes needed?

I was looking into database pool configuration and found a red herring in flyte admin.

What changes were proposed in this pull request?

Remove unused configuration fields.

How was this patch tested?

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• I updated the documentation accordingly.
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.22%. Comparing base (433d1a0) to head (36e6640).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7644      +/-   ##
==========================================
- Coverage   57.22%   57.22%   -0.01%     
==========================================
  Files         930      930              
  Lines       58196    58190       -6     
==========================================
- Hits        33303    33297       -6     
  Misses      21840    21840              
  Partials     3053     3053              

Flag	Coverage Δ
unittests-datacatalog	53.51% <ø> (ø)
unittests-flyteadmin	53.23% <ø> (-0.02%)	⬇️
unittests-flytecopilot	48.05% <ø> (ø)
unittests-flytectl	64.09% <ø> (ø)
unittests-flyteidl	75.71% <ø> (ø)
unittests-flyteplugins	60.38% <ø> (ø)
unittests-flytepropeller	53.78% <ø> (ø)
unittests-flytestdlib	64.39% <ø> (ø)

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