github/workflows: delete grpcproxy workflow

[ivanvc]

These jobs are already running on the Prow infrastructure. We should be able to remove them to declutter the checks run in every pull request.

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.94%. Comparing base (49fbd1e) to head (b1ecb20).
Report is 8 commits behind head on main.

Additional details and impacted files

see 22 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19496      +/-   ##
==========================================
+ Coverage   68.87%   68.94%   +0.07%     
==========================================
  Files         420      420              
  Lines       35788    35788              
==========================================
+ Hits        24648    24674      +26     
+ Misses       9714     9686      -28     
- Partials     1426     1428       +2     

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ivanvc

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[ivanvc]

/cherry-pick release-3.6

[k8s-infra-cherrypick-robot]

@ivanvc: new pull request created: #19551

In response to this:

/cherry-pick release-3.6

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