Remove outdated LibGit2.get_creds! methods

[omus]

Part of #20725.

Did a cleanup pass after #23189.

• Moved the AbstractCredentials closer to the other credentials. Previously this code needed to be defined before a RemoteCallbacks constructor that no longer exists.
• Deprecated Remove some get_creds! methods which are no longer required since the introduction of the CredentialPayload.

[omus]

I'm not sure what the protocol is when we're removing a method with no replacement. I'm assuming it should still go through a deprecation cycle.

[Keno]

If it wasn't exported, it doesn't need a deprecation cycle.

[omus]

Unfortunately most things in the LibGit2 module aren't exported so it can be difficult to know what is being used externally. I'm pretty sure these methods are not be used externally so I'll remove the "base/deprecated.jl" changes.

[omus]

Removed the deprecations.

[tkelman]

What's the "too many prompts" error on circle about?

Agreed that if things are externally used then it's nice as a courtesy to deprecate instead of hard break, when possible. If not externally used then it's not too important.

[omus]

What's the "too many prompts" error on circle about?

That is coming from the challenge_prompt and typically only appears during a LibGit2 test failure. It is possible however that this is a false positive as the way we detect "too many prompts" by using a timeout of 10 seconds.

Is there a reason there isn't a full traceback on Circle CI?

[omus]

I've triggered a rebuild to see if it reoccurs. The failing Circle CI log can be seen here.

[tkelman]

dunno, would we get a better backtrace on that kind of failure on travis? it might be something strange with piping or where stderr goes, or it may relate to #18647?

[omus]

On Travis CI I typically see full stack traces for those kind of exceptions. I'm not going to worry too much about it as it's only occurred once.

Any other comments?

[omus]

Removed one more unused method. I also changed the first commit message as it referred to deprecating a method instead of removing it.