Remove pkg/ready

[hasbro17]

Description of the change:
Remove the readiness probe helpers in pkg/ready.

Motivation for the change:
With the new scaffolds the readiness probe helpers in pkg/ready are no longer used by default, and need to be removed from the main CLI repo for the v1.0.0 release.
Related to #3327

TODO

• Add a new changelog fragment

[camilamacedo86]

They are no longer used by default. However, are still they valid/useful with the new layout? Would we really like to no longer support it? If yes, we need to add a fragment with the info for the migration guide since it is a significant breaking change. See that it has been used: https://github.com/search?q=operator-sdk%2Fpkg%2Fready&type=Code

[hasbro17]

@camilamacedo86 Per our pervious discussions we decided the readiness probe helpers would be removed in v1.0.0.
It made sense when they were part of our scaffolds but the new plugin scaffolds don't stipulate using readiness probes by default.

And for users who still want it, the helpers are only a few lines of writing/reading a file on disk. Should be easy enough to replicate.

And yes I have a TODO for the fragment which I'll add shortly.

[camilamacedo86]

Just missing fragment. Otherwise, shows ok.
/lgtm

[jmrodri]

/lgtm

[joelanford]

Just for reference, we removed usage of the ready package from the Go scaffold all the way back in v0.4.0 because the way we implemented it caused deadlock problems. See #932

Also controller-runtime supports running a readyz server with custom http handlers, which is what users should use going forward. Simply add a healthz.Checker (e.g. healthz.Ping) using manager.AddReadyzCheck