Remove extra if block in calculate_chunk that was added due to Koalas bug

### Remove extra if block in calculate_chunk that was added due to Koalas bug

During the Woodwork integration, a code block was added to address an issue where appending Koalas dataframes could cause the Woodwork schema to be invalidated as a result of Koalas changing the dtypes of the columns unexpectedly.

[Koalas Issue 2193](https://github.com/databricks/koalas/issues/2193) was created to resolve this issue. Once the Koalas issue is closed and a fix is released, we should be able to delete the code block shown below from the `calculate_chunk` method in `calculate_feature_matrix.py`.

```python
if not fm.ww.schema:
# Koalas bug sometimes causes schema to be invalidated
# after `append` so need to reinitialize WW
fm.ww.init(**ww_init_kwargs)
```