Check if we can remove PVVolumeSnapshotContentList

[xing-yang]

What happened:
Check if we can remove PVVolumeSnapshotContentList []PVVolumeSnapshotContentPair from the APIs. This came up during the KEP review.

What you expected to happen:

How to reproduce it:

Anything else we need to know?:

Environment:

• Driver version:
• Kubernetes version (use kubectl version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others: