velero should not accept requests to delete backups from ReadOnly BackupStorageLocations

[puneetkatyal]

What steps did you take and what happened:
[A clear and concise description of what the bug is, and what commands you ran.)

• Add a ReadOnly backup location
• Attempt to delete one of the backups from this location using the velero backup delete <backup_name> command
• velero accepts the backup deletion request without throwing an error, even though the actual backup is not deleted. However, it is misleading.

$ velero backup delete backup-5
Are you sure you want to continue (Y/N)? Y
Request to delete backup "backup-5" submitted successfully.
The backup will be fully deleted after all associated data (disk snapshots, backup files, restores) are removed.

What did you expect to happen:
I expect to receive an error if I attempt to delete a backup from a ReadOnly BackupStorageLocation

The output of the following commands will help us better understand what's going on:
(Pasting long output into a GitHub gist or other pastebin is fine.)

• kubectl logs deployment/velero -n velero
• velero backup describe <backupname> or kubectl get backup/<backupname> -n velero -o yaml
• velero backup logs <backupname>
• velero restore describe <restorename> or kubectl get restore/<restorename> -n velero -o yaml
• velero restore logs <restorename>

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]
The server I'm testing with is running with --restore-only option.

Environment:

• Velero version (use velero version): v1.7.0-rc.1
• Velero features (use velero client config get features): EnableAPIGroupVersions
• Kubernetes version (use kubectl version): 1.19.9
• Kubernetes installer & version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release): velero client is running on MacOS 11.5.2; Kubernetes cluster is running Ubuntu 20.04 nodes.

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