groupby: Don't set method by default on flox>=0.9
#11160
| Job | Run time |
|---|---|
| 5s | |
| 3s | |
| 7m 49s | |
| 8m 19s | |
| 7m 59s | |
| 11m 42s | |
| 21m 38s | |
| 16m 6s | |
| 14m 22s | |
| 13m 34s | |
| 15m 4s | |
| 1m 30s | |
| 6m 49s | |
| 5m 53s | |
| 7m 53s | |
| 2h 18m 46s |