Fix onefuzz repro bug - Remove managed identity from CustomScriptExtension

[nharper285]

Summary of the Pull Request

What is this about?
The CustomScriptExtension began failing due to the empty managed identity that's added to its configuration. Despite having this configuration for some time, the extension began failing abut three weeks ago (potentially due to an internal change).

This PR removes the managed identity from the extension config.

PR Checklist

• Applies to work item: CustomScriptExtension fails to provision during repro startup.  #2919
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

What does this include?

Validation Steps Performed

How does someone test & validate?

[mgreisen]

            ProtectedSettings = new BinaryData(managedIdentity)

don't we need to do this here as well?

---

Refers to: src/ApiService/ApiService/onefuzzlib/Extension.cs:318 in bb16de8. [](commit_id = bb16de8, deletion_comment = False)

[codecov-commenter]

Codecov Report

Merging #2920 (ab3a4fc) into main (f00248f) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #2920   +/-   ##
=======================================
  Coverage   28.84%   28.84%           
=======================================
  Files         302      302           
  Lines       35895    35895           
=======================================
  Hits        10353    10353           
  Misses      25542    25542           

Impacted Files	Coverage Δ
src/ApiService/ApiService/onefuzzlib/Extension.cs	0.00% <0.00%> (ø)
...piService/ApiService/onefuzzlib/ReproOperations.cs	10.81% <ø> (ø)
...c/ApiService/ApiService/onefuzzlib/VmOperations.cs	0.00% <ø> (ø)

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