github: remove amd64 test actions

[ivanvc]

The pull-etcd-unit-test-amd64, pull-etcd-unit-test-386, pull-etcd-integration-1-cpu-amd64, pull-etcd-integration-2-cpu-amd64, and pull-etcd-integration-8-cpu-amd64 jobs are already running on the prow infrastructure. It is safe to remove these duplicated workloads from GitHub actions.

Fixes #18065
Part of kubernetes/k8s.io#6102

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[ivanvc]

Given that now the tests-template.yaml is now only used by arm64, does it make sense to remove it? Or should we leave it as is, as we'll eventually remove this when we have prow arm64 runners?

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.93%. Comparing base (bc9801f) to head (487af49).

❗ Current head 487af49 differs from pull request most recent head 6325cce

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Additional details and impacted files

see 18 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18314      +/-   ##
==========================================
- Coverage   68.96%   68.93%   -0.03%     
==========================================
  Files         417      417              
  Lines       35332    35332              
==========================================
- Hits        24366    24356      -10     
- Misses       9554     9558       +4     
- Partials     1412     1418       +6     

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[jmhbnz]

/lgtm
/approve

Thanks @ivanvc

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