cli/command: remove deprecated RegistryAuthenticationPrivilegedFunc

[thaJeztah]

• relates to cli/command: deprecate RegistryAuthenticationPrivilegedFunc #6184

This function was deprecated in 29263e8 and is no longer used.

- Human readable description for the release notes

Go SDK: cli/command: remove deprecated `RegistryAuthenticationPrivilegedFunc`

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

👋 @nicksieger @nicks (not sure who has most info and time); we started on revamping authentication, and migrating Moby to proper go modules, but as part of that we're first trying to, slightly faster than normal, remove as much legacy "cruft" as possible; the RegistryAuthenticationPrivilegedFunc is part of that, but looking at potential consumers, it looks like Tilt is the only (visible / public) user of this function outside of the CLI itself.

Wondering what exactly Tilt needs, and if some replacement (which could be "copy/paste forking some bits") is needed for Tilt. (also /cc @Benehiko who may also be able to help).

Here's where I found it in tilt-dev;
https://github.com/tilt-dev/tilt/blob/ecbb943be29bd7d88dc2623830d9a3943ca62c06/internal/docker/client.go#L405-L411
https://github.com/tilt-dev/ctlptl/blob/ed6a3f888eba0c13d3cf6a9c0f0b74533b8c82fe/internal/dctr/run.go#L55-L64

related;

• docker: pass credentials to image pull tilt-dev/ctlptl#306

[nicksieger]

Looks like Tilt just used the function as part of the previous approach for getting registry credentials. Tilt isn't doing anything special, so when Tilt upgrades to the latest version of docker/cli that removes this, it will need to be removed from Tilt also; just a normal upgrade process.