Cleanup code for feature-gates HighAvailability, SelfHosting, CertsInSecrets

[fabriziopandini]

kubernetes/kubernetes#67786 deprecated feature-gates HighAvailability, SelfHosting, CertsInSecrets in v1.12, so in v1.13 cycle we could cleanup code from HighAvailability, SelfHosting, CertsInSecrets implementation.

As discussed in kubeadm office hours the 22 August 2018, we should consider if to preserve the pivoting logic (currently still accessible as kubeadm alpha phase self hosting) into a separated sub command + eventually implement also un-pivoting

[fabriziopandini]

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[Klaven]

Talked about this in the kubeadm office hours on Wed 3 Oct 2018. the high level overview of the work that should be done is as follows:
We will not remove the feature, instead we will move it too it's own command and add a warning about it's limitations and restrictions.

[neolit123]

closed by:
kubernetes/kubernetes#69878