storage: remove historic information for CSI inline volumes

As pointed out during review, this information is better captured in
the feature gates reference page.

content/en/docs/concepts/storage/volumes.md

@@ -1315,7 +1315,6 @@ Learn how to
 
 #### CSI ephemeral volumes
 
-{{< feature-state for_k8s_version="v1.15" state="alpha" >}}
 {{< feature-state for_k8s_version="v1.16" state="beta" >}}
 
 This feature allows CSI volumes to be directly embedded in the Pod specification instead of a PersistentVolume. Volumes specified in this way are ephemeral and do not persist across Pod restarts.
@@ -1344,12 +1343,7 @@ spec:
 ```
 
 This feature requires CSIInlineVolume feature gate to be enabled. It
-is enabled by default in Kubernetes >= 1.16. In Kubernetes v1.15, it
-has to be enabled explicitly:
-
-```
---feature-gates=CSIInlineVolume=true
-```
+is enabled by default starting with Kubernetes 1.16.
 
 CSI ephemeral volumes are only supported by a subset of CSI drivers. Please see the list of CSI drivers [here](https://kubernetes-csi.github.io/docs/drivers.html).