Since the only member of that struct is gone, the struct itself can also be
removed. If for whatever reason the struct is needed again, then this commit
can be reverted to bring it back.
The feature gate gets locked to "true", with the goal to remove it in two
releases.
All code now can assume that the feature is enabled. Tests for "feature
disabled" are no longer needed and get removed.
Some code wasn't using the new helper functions yet. That gets changed while
touching those lines.
It's not enough to silently drop the volume type if the feature is
disabled. Instead, the policy should fail validation, just as it would
have if the API server didn't know about the feature at all.
