excel header_style broken in 0.22

```python
import pandas as pd
pd.io.formats.excel.header_style = None
```

After updating, it is no longer possible to override the excel header styles.

Previously, this was possible in 0.19.2:

`pd.formats.format.header_style = None`

It seems header_style has been moving around constantly lately... Can we please put it somewhere and leave it there??? Or, if it's going to be moved constantly, it should live on a private module path. I realize python doesn't have private scoping, but there are paths in the pandas module that are privately scoped, by convention, yet don't follow the standard python convention of underscoring the names.

How are we supposed to accomplish the removal of styles on the latest build?

That module path DOES indeed exist... but something is wonky with the imports, it seems.

Exception is raised:
`AttributeError: 'module' object has no attribute 'excel' `

Possibly related? https://github.com/pandas-dev/pandas/pull/15530