Simplify `to_parquet` compute path (#8982)
Simplify `to_parquet` compute path (#8982)

Previously `to_parquet` would either create a new `Scalar` or construct
and compute a graph directly. Due to slight differences between these
code paths a user could see different scheduling and performance
behavior between:

```
df.to_parquet()
df.to_parquet(compute=False).compute()
```

To fix this, we remove the branch and make these equivalent statements.
jakirkham/dask,dask/dask,jakirkham/dask,dask/dask