BUG: Subplotting boxplot shows unnecessary warnings
Related to #8877 and 80a730c93717e7fc01ae2f880109bc752519cecf

Because `legend=True` is default, `DataFrame.plot(kind='box', subplots=True)` shows unnecessary warnings.

```
# OK, no legend / no warnings
df.plot(kind='box')
```

```
# NG: no legend and warnings
df.plot(kind='box', subplots=True)
# UserWarning: No labelled objects found. Use label='...' kwarg on individual plots.
#   warnings.warn("No labelled objects found. "
```
