chore: minor endpoint cleanup

[krancour]

Working on resolving merge conflicts in #1566, I noticed some weirdness in existing endpoints.

This PR:

• Removes unnecessary gets before deletes.
• Removes unnecessary wrapping of Kubernetes "not found" errors as connect "not found" errors. Our error interceptor is already smart enough to do that. We don't need to do it explicitly.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 21.62%. Comparing base (8534a0c) to head (47d0292).

Files	Patch %	Lines
internal/api/delete_stage_v1alpha1.go	0.00%	9 Missing ⚠️
internal/api/delete_warehouse_v1alpha1.go	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1608      +/-   ##
==========================================
+ Coverage   21.61%   21.62%   +0.01%     
==========================================
  Files         188      188              
  Lines       27936    27912      -24     
==========================================
  Hits         6037     6037              
+ Misses      21620    21596      -24     
  Partials      279      279              

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