docker info: don't print "registry" (IndexServerAddress)

[thaJeztah]

• fixes docker info reports an index server url as a registry url #3793
• relates to registry: don't call "/info" API endpoint to get default registry #2819
• relates to cli/command: remove deprecated ElectAuthServer() #4110

The IndexServerAddress field was as part of the initial Windows implementation of the engine. For legal reasons, Microsoft Windows (and thus Docker images based on Windows) were not allowed to be distributed through non-Microsoft infrastructure. As a temporary solution, a dedicated "registry-win-tp3.docker.io" registry was created to serve Windows images.

Currently, this field always shows "https://index.docker.io/v1/", which is confusing, because that address is not used for the registry (only for authentication and "v1" search).

docker info
...
Registry: https://index.docker.io/v1/

Starting with b4ca1c7, this field is also no longer used during authentication, and a3d56e7 removed the (deprecated) ElectAuthServer() which was previously used to query it.

Given that there's currently no practical use for this information, and it only adds "noise" (and confusion), this patch removes it from the default output.

For now, the field is (still) available for those that want to use it;

docker info --format '{{.IndexServerAddress}}'
https://index.docker.io/v1/

But it won't be printed by default.

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[thaJeztah]

@vvoland @rumpl ptal /cc @oyabun @dvdksn (we need to make a pass through docs elsewhere to remove it in examples (if any)

[codecov-commenter]

Codecov Report

Merging #4204 (375b17a) into master (c25115e) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4204      +/-   ##
==========================================
- Coverage   58.86%   58.86%   -0.01%     
==========================================
  Files         572      572              
  Lines       49546    49544       -2     
==========================================
- Hits        29164    29162       -2     
  Misses      18616    18616              
  Partials     1766     1766              

[dvdksn]

We can remove the docs/revisit label as docker/docs#17106 takes care of this.

[thaJeztah]

Thanks!