REF: remove unused allow_object kwarg from sequence_to_dt64ns
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

@tacaswell looks like we may need to update some of our matplotlib code?  Any suggestions?

```
    @pytest.mark.slow
    def test_axis_share_x_with_by(self):
        # GH 15079
        ax1, ax2, ax3 = self.hist_df.plot.hist(column="A", by="C", sharex=True)
    
        # share x
>       assert ax1._shared_x_axes.joined(ax1, ax2)
E       AttributeError: 'AxesSubplot' object has no attribute '_shared_x_axes'
```