TidbCluster: delete failure component Pod

[LinuxGit]

Feature Request

Describe the feature you'd like:

eg. One of the Pod tikv-pod-1 in the tidb cluster failed (the K8s node was down and can't recover), TiDB Operator failover and start a new tikv-pod-3, we want to remove tikv-pod-1.

[tennix]

There are two options to accomplish this:

• Change the failover logic to delete the old TiKV and create a new TiKV to replace it instead of creating another ordinal TiKV
• Support scale in pod at arbitrary ordinal for statefulset.

The first needs to forcibly delete Pod/PVC/Store, it has risk of losing data.

@cofyc has implemented a modified statefulset controller that allows scale in at arbitrary ordinal.