generic ephemeral volumes: completed in 1.23

All remaining code and doc changes went into 1.23. All code behind feature gate
checks and the check themselves were removed.

Author: pohly

keps/sig-storage/1698-generic-ephemeral-volumes/kep.yaml

@@ -5,7 +5,7 @@ authors:
 owning-sig: sig-storage
 participating-sigs:
   - sig-storage
-status: implementable
+status: implemented
 creation-date: 2020-04-17
 reviewers:
   - "@jsafrane"