Retain volumesnapshot-being-created annotation until readyToUse=true

[pwschuurman]

What type of PR is this?
/kind bug

What this PR does / why we need it:
In createSnapshotWrapper(), the snapshot.storage.kubernetes.io/volumesnapshot-being-created annotation is removed even if the snapshot is not marked as readyToUse true. This is inconsistent with the same logic in syncContent() where we only remove the annotation if the snapshot content is ready:

external-snapshotter/pkg/sidecar-controller/snapshot_controller.go

Line 102 in 59d7297

_, err = ctrl.removeAnnVolumeSnapshotBeingCreated(content)

Which issue(s) this PR fixes:

This change may reduce symptoms seen in #1282. However this does not fix the root cause of immedate vs. scheduled exponential backoff requeuing.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Remove VolumeSnapshotContent ``snapshot.storage.kubernetes.io/volumesnapshot-being-created annotation only when status.readyToUse is true

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pwschuurman
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