Remove deprecated splitBrainThresholdSeconds

[kuritka]

The splitBrainThresholdSeconds value was implemented a long time ago for Infoblox but has since been deprecated. We removed all related implementation, although the splitBrainThresholdSeconds field is still part of the GSLB spec and CRD. The reason for keeping it is that clients may have this value defined in their GSLB, and if we removed it from the CRD, k8gb would stop working for them after k8gb upgrade. Removing splitBrainThresholdSeconds from CRD will require migration configuration.

The purpose of this ticket is to track splitBrainThresholdSeconds and eventually remove it when we release k8gb with multiple breaking changes.

see #1829.