Remove built-in JSch implmentations

[Dohbedoh]

(amends #19)

• The built-in authenticators have been moved to jsch plugin a long time ago Merge JSch auth classes and bump dependencies jsch-plugin#2.
• There does not seem to be any usage of the JSchConnector of the ssh-credentials plugin anywhere https://github.com/search?q=user%3Ajenkinsci+JSchConnector&type=Code.

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