Remove mention of SSH passphrase, it was only present in v0.12

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #6605 from SvenDowideit/6423-remove-b2d-ssh-passphrase-prompt

Remove mention of SSH passphrase, it was only present in v0.12