Make clarification on Checkout Credential configuration in USER_GUIDE

[TeCHiScy]

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Mention that the Checkout Credential configuration was removed, and the alternative approach to configure a SSH credential for checkout in USER_GUIDE.

closes #715

[KalleOlaviNiemitalo]

Because the change was made 7 years ago, I think it's no longer necessary to describe the old UI.

[TeCHiScy]

Because the change was made 7 years ago, I think it's no longer necessary to describe the old UI.

Revised as suggested.