Remove 'Private Distributor List' from security guide

[ahrtr]

We don't have such a list for now, so remove it to avoid any confusion

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 68.88%. Comparing base (be63721) to head (b7250f6).

Additional details and impacted files

see 38 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21012      +/-   ##
==========================================
- Coverage   69.24%   68.88%   -0.36%     
==========================================
  Files         422      419       -3     
  Lines       34841    34706     -135     
==========================================
- Hits        24124    23908     -216     
- Misses       9318     9372      +54     
- Partials     1399     1426      +27     

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[spzala]

Makes sense.
lgtm, thanks @ahrtr !!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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