[cleanup] Remove unused files and unrelated logic from /api

[swang392]

What does this PR do?

• removes datadogagent_conversion.go and datadogagent_webhook.go
• moves api/datadoghq/v2alpha1/secret.go to pkg/secrets/secrets.go

Motivation

What inspired you to submit this pull request?

Additional Notes

Anything else we should know when reviewing?

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

Describe your test plan

should have no change

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 17.18750% with 53 lines in your changes missing coverage. Please review.

Project coverage is 48.71%. Comparing base (463ab54) to head (eb8a787).

Files with missing lines	Patch %	Lines
pkg/secrets/secrets.go	0.00%	50 Missing ⚠️
...ller/datadogagent/feature/enabledefault/feature.go	50.00%	1 Missing ⚠️
...er/datadogagent/feature/eventcollection/feature.go	66.66%	1 Missing ⚠️
...er/datadogagent/feature/externalmetrics/feature.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1620      +/-   ##
==========================================
- Coverage   48.82%   48.71%   -0.12%     
==========================================
  Files         217      217              
  Lines       20882    20932      +50     
==========================================
  Hits        10196    10196              
- Misses      10149    10199      +50     
  Partials      537      537              

Flag	Coverage Δ
unittests	48.71% <17.18%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...controller/datadogagent/component/agent/default.go	79.35% <100.00%> (ø)
...controller/datadogagent/controller_reconcile_v2.go	51.89% <100.00%> (ø)
pkg/controller/utils/datadog/metrics_forwarder.go	31.74% <100.00%> (ø)
...ller/datadogagent/feature/enabledefault/feature.go	36.62% <50.00%> (ø)
...er/datadogagent/feature/eventcollection/feature.go	62.31% <66.66%> (ø)
...er/datadogagent/feature/externalmetrics/feature.go	54.16% <80.00%> (ø)
pkg/secrets/secrets.go	41.53% <0.00%> (-25.97%)	⬇️

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