prune unused dockerfiles & build targets

[grokspawn]

Description of the change:
Removes a couple of dockerfiles which aren't currently used in build/release processes. We can resurrect them in future if we need to.
Removes a build target which refers to a dockerfile which isn't even present in the repo.

Motivation for the change:
Replaces #1914

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[Copilot]

Pull request overview

This PR cleans up container build artifacts by removing Dockerfiles and a Makefile build target that are not used by the current build/release workflows, reducing maintenance surface and confusion around supported image builds.

Changes:

• Delete upstream-opm-builder.Dockerfile (deprecated builder Dockerfile).
• Delete registry.Dockerfile (standalone registry-server image Dockerfile).
• Remove the image-upstream Makefile target that builds upstream-example.Dockerfile.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
upstream-opm-builder.Dockerfile	Removed unused/deprecated Dockerfile.
registry.Dockerfile	Removed standalone registry-server Dockerfile.
Makefile	Removed unused image-upstream build target.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.71%. Comparing base (12f8cdd) to head (18ff135).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1961   +/-   ##
=======================================
  Coverage   57.71%   57.71%           
=======================================
  Files         139      139           
  Lines       13373    13373           
=======================================
  Hits         7718     7718           
  Misses       4468     4468           
  Partials     1187     1187           

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[tmshort]

/lgtm

[tmshort]

/approve

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tmshort

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