ARUHA-242: Remove validation_strategies from the API yaml.
ARUHA-242: Remove validation_strategies from the API yaml.

This removes the `validation_strategies` property from event
type, and the corresponding registry path to see validations
available in the installation. These features are not
implemented yet by the server, and they can be added back
when they're available.

For #287

zalando/nakadi,zalando/nakadi