Remove appregistry-related tooling

[ecordell]

Description of the change:
Removes appregistry-server and related code

Motivation for the change:
Appregistry-server was only for openshift support, and hasn't been supported in openshift since 4.5 (deprecated in 4.4).

If there are any straggling appregistry-server users, the existing published binaries should continue to work as long as appregistry is available (the protocol itself is EOLing soon), and should the need arise the appregistry-server can be maintained on existing release branches.

Reviewer Checklist

• Implementation matches the proposed design, or proposal is updated to match implementation
• Sufficient unit test coverage
• Sufficient end-to-end test coverage
• Docs updated or added to /docs
• Commit messages sensible and descriptive

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ecordell

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• OWNERS [ecordell]

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[codecov]

Codecov Report

Merging #608 (5ebd6d1) into master (4b5d403) will increase coverage by 1.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
+ Coverage   48.80%   50.59%   +1.78%     
==========================================
  Files          90       68      -22     
  Lines        6276     5483     -793     
==========================================
- Hits         3063     2774     -289     
+ Misses       2528     2097     -431     
+ Partials      685      612      -73     

Impacted Files	Coverage Δ
pkg/mirror/options.go	0.00% <ø> (ø)

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[joelanford]

/lgtm