[Enhancement] Drop support of passing credentials to etcd-backup-restore through a JSON file

[renormalize]

Enhancement (What you would like to be added):

Currently there are two ways of passing credentials to etcd-backup-restore, which are:

• As individual fields in a Secret (consumed as individual files for each field in a directory, by etcd-backup-restore).
• As a JSON data field in a Secret (consumed as a JSON file, by etcd-backup-restore).

The intent is to drop support to pass credentials through the JSON format, and only support individual fields in a Secret (consumed as individual files in a directory by etcd-backupr-restore).

Motivation (Why is this needed?):

There was confusion in the community regarding how credentials are passed to etcd-backup-restore, and it was decided to only allow one way of passing credentials, as discussed in #729 (comment).

Approach/Hint to the implement solution (optional):

Remove all code that enables the passing of credentials as a JSON file in etcd-backup-restore.