Remove dead code after dropping support for k8s 1.16 and earlier

[croomes]

Removes some complexity that was required for earlier versions of Kubernetes to select appropriate images. From k8s 1.15 onwards, image selection is static, apart from adding the csi-resizer from k8s 1.16 onwards.

This helps avoid confusion about which images should be pre-loaded into private registries.

Also removes the NFS controller that was used in StorageOS v1.

There are no changes to the StorageOSCluster CR so upgrades will work as before. Attempting to run the operator on k8s 1.14 or earlier will fail with an error: kubernetes 1.15.0 or above required.

@avestuk has agreed to test across the supported platforms prior to merge.