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. "
```

Looking at the travis doc build, the warning is still there.
 Looks the same warning is raised by hexbin also. Let me fix.
 Looks fixed. Other warnings are unrelated to this change, and being fixed in #8877.

https://travis-ci.org/pydata/pandas/jobs/47391584
 Great. It does look like all the no labelled objects errors have been fixed.

Thanks!
