apiserver: remove support for old login API versions

[rogpeppe]

And remove some other redundant code relating
to API compatibility too.

To QA, bootstrap a controller and check that you can log into it.

(Review request: http://reviews.vapour.ws/r/5439/)

[rogpeppe]

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[rogpeppe]

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[rogpeppe]

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[rogpeppe]

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[rogpeppe]

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[rogpeppe]

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[rogpeppe]

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[rogpeppe]

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