br: `check-requirements=false` shoud only skip check version and br should always check whether target tidb is empty

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1. `br full restore` to non-empty tidb cluster with `check-requirements=false`

### 2. What did you expect to see? (Required)
1.report error , non-empty tidb shouldn't be target , and restore should exit
### 3. What did you see instead (Required)
1. no error before restore
2. restore to cluster success
3. cluster data is not consistent
4. docs and /br --help just mention `check-requirements=false` skip version check
### 4. What is your TiDB version? (Required)
tidb: master
br: v6.4.0