Refs MW-252

- reverted from tracking donor servicing thread. With xtrabackup SST,
  xtrabackup thread will call FTWRL and node is desynced upfront
- Skipping desync in FTWRL if node is operating as donor

sql/sql_class.h

@@ -3880,7 +3880,6 @@ public:
   bool                      wsrep_apply_toi; /* applier processing in TOI */
   bool                      wsrep_skip_append_keys;
   wsrep_gtid_t              wsrep_sync_wait_gtid;
-  my_bool                   wsrep_donor; /* true if thread is SST donor servicing */
   ulong                     wsrep_affected_rows;
 #endif /* WITH_WSREP */
 };