BUG: xticks unnecessarily rotated
- [x] closes #29460
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

On `master`, xticklabels are rotated if
```python
            not self._use_dynamic_x()
            and data.index.is_all_dates
            and not self.subplots
            or (self.subplots and self.sharex)
```

There's a comment saying
```python
        if condition:
            # irregular TS rotated 30 deg. by default
            # probably a better place to check / set this.
            if not self._rot_set:
                self.rot = 30
```
, and it seems that the _intention_ was to rotate ticklabels if there's an irregular timeseries.

With the condition as it is written, they will also be rotated if the following is true:
```python
(self.subplots and self.sharex)
```

I don't see why that should warrant rotating the ticklabels, so I'd like to suggest that there was a bracketing error.

Also, the condition `data.index.is_all_dates` is (as far as I can tell) only relevant if `use_index=True`, so I've changed that.

Here's what the OP's plot looks like with these changes:

![image](https://user-images.githubusercontent.com/33491632/82729010-04810280-9cec-11ea-825c-5244a2a1f828.png)


Here's what it looks like on master:

![image](https://user-images.githubusercontent.com/33491632/82729022-1bbff000-9cec-11ea-9954-4e75488f91b8.png)

Irregular time series still have their ticklabels rotated:
![image](https://user-images.githubusercontent.com/33491632/84986706-e796ee00-b136-11ea-9df1-d067a0de134b.png)

Regular time series' ticklabels don't rotate:
![image](https://user-images.githubusercontent.com/33491632/84986756-fd0c1800-b136-11ea-857d-738f17bb74c1.png)

(unless you pass `xcompat=True`, but that's unaffected by this change)
