Should use python.pythonPath setting

As Microsoft recommends its users to use the Python extension from itself,

https://code.visualstudio.com/docs/languages/python

It is more common for a project to use the `pythonPath` setting in `python` section such as,
```
{
    "python.pythonPath": "/usr/local/bin/python3",
}
```
`rst.preview.pythonPath` seems to be duplicate.