ci: cleaning up references to skaffold slim as it is no longer used

[Darien-Lin]

Description
The cloud build trigger for the slim build has been failing for over a year now. There is no longer a need to have a slim build, so removing the definitions and machinery around it.

[code-review-assist]

Hi team,

I'm currently reviewing this pull request and will post my full review in a few minutes. In the meantime, here's a quick summary for you and other reviewers to get up to speed:

This PR removes the now-unused "slim" build of Skaffold. The cloud build trigger for the slim build has been broken for over a year, and there's no longer a need for it. The changes involve:

• Removing the release-slim target from the Makefile (lines 184-191).
• Deleting the deploy/cloudbuild-slim.yaml file entirely (this file was entirely removed).
• Removing the deploy/skaffold/Dockerfile.slim file entirely (this file was entirely removed).

The intent is to simplify the build process and remove unnecessary complexity. This should improve maintainability and reduce potential issues.

---

Here's a little haiku to lighten the mood:

Slim build's gone away,
Cleaner code, a brighter day,
Skaffold now is lean.

Thanks,
[Your Name]

[code-review-assist]

This pull request removes the slim build configuration which seems to be no longer necessary. Removing unused code and configurations is a good practice and improves maintainability. The provided context is helpful in understanding the reason for these changes. Overall, the changes look good. A few minor suggestions follow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.14%. Comparing base (290280e) to head (71c632d).
Report is 1228 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9615      +/-   ##
==========================================
- Coverage   70.48%   63.14%   -7.34%     
==========================================
  Files         515      648     +133     
  Lines       23150    33607   +10457     
==========================================
+ Hits        16317    21221    +4904     
- Misses       5776    10729    +4953     
- Partials     1057     1657     +600     

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