raft: skip mts_recovery_groups during start slave

Summary:
During MySQL8+Raft DMP, some instance fail to switch to Leader or start slave

```
2021-06-24T17:56:38.627423-07:00 431 [Note] [MY-010574] [Repl] Slave: MTS group recovery relay log info group_master_log_name /data/mysql/3127/bls-unittestdb658.frc2-3305-mysql.replicaset.180021/binary-logs-3727.000033, event_master_log_pos 1129.
2021-06-24T17:56:38.627473-07:00 431 [ERROR] [MY-010575] [Repl] Error looking for file after /binlogs/binary-logs-3307.000120.
2021-06-24T17:56:38.627516-07:00 431 [ERROR] [MY-000000] [Repl] load_mi_and_rli_from_repositories: rli_init_info returned error
```

similar to 5.6, we don't need to run mts_recovery_groups due to GTID_MODE is always enabled.

Reviewed By: Pushapgl

Differential Revision: D29520066

fbshipit-source-id: d3b9fe21d84

sql/rpl_slave.cc

@@ -6596,6 +6596,10 @@ bool mts_recovery_groups(Relay_log_info *rli) {
     return false;
   }
 
+  // raft replication always have GTID_MODE=ON, thus ignore positions
+  if (enable_raft_plugin) {
+    return false;
+  }
   /*
     Save relay log position to compare with worker's position.
   */