Clean up CFG_NODE_COUNT where it's not needed

[nwnt]

I'm removing CFG_NODE_COUNT where it's no longer needed, especially those passed as a flag during the build time. The flag is now only needed for selecting which docker compose file to use (either cluster of 1 or 3 nodes).

Also ran the following targets and confirmed they were successful:

• make antithesis-build-etcd-image
• make antithesis-build-client-docker-image
• make antithesis-docker-compose-up
• make antithesis-run-container-traffic
• make antithesis-run-container-validation

[k8s-ci-robot]

@nwnt: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-integration-2-cpu-arm64	16d6ff7	link	true	/test pull-etcd-integration-2-cpu-arm64

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.22%. Comparing base (855c9ba) to head (16d6ff7).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

see 24 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #20868      +/-   ##
==========================================
+ Coverage   69.19%   69.22%   +0.03%     
==========================================
  Files         422      422              
  Lines       34822    34822              
==========================================
+ Hits        24095    24106      +11     
+ Misses       9331     9322       -9     
+ Partials     1396     1394       -2     

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[nwnt]

forgot to cc @serathius @joshjms

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nwnt, serathius

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