Remove snapshot e2e testing from alpha versions.

[msau42]

What type of PR is this?
//kind failing-test

What this PR does / why we need it:
Disables snapshot e2es for 1.16 deployment.

prow.sh uses the e2e version that matches the k8s version. Skew testing
(driver version != k8s version) doesn't work for alpha features.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[msau42]

/assign @xing-yang

[msau42]

Failing testgrid: https://k8s-testgrid.appspot.com/sig-storage-csi-ci#1.16-on-1.17

[xing-yang]

Should we also remove "snapshotter" folder and "hostpath/csi-hostpath-snapshotter.yaml" under kubernetes-1.15 and kubernetes-1.16?

[msau42]

Should we also remove "snapshotter" folder and "hostpath/csi-hostpath-snapshotter.yaml" under kubernetes-1.15 and kubernetes-1.16?

I think people may still want to deploy hostpath driver on older versions. We just can't test it in our e2es

[xing-yang]

/lgtm
/approve

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