Remove generator option from book.json

If you look at [the README of gitbook](https://github.com/GitbookIO/gitbook#how-to-use-it), it says:

```// Generator to use for building```
```// Caution: it overrides the value from the command line```
```// It's not advised this option in the book.json```
```"generator": "site",```

For example, if I want to run the ```gitbook pdf```command to convert the gitbook to a PDF, it doesn't work when the generator option is specified to be ```site``` in the book.json because book.json will override the value from the command line (for example, ```pdf```) with ```site``` instead.

So I get the following error:
```Starting build ...```
```ENOENT, lstat '/var/folders/ld/z9_z78d11m57k250mryfnx_h0000gn/T/tmp-37086ama1eyo/index.pdf'```

The way to fix this is to simply remove the generator option from book.json. So until now I have done it manually whenever I want to convert the gitbook to a PDF. But then other people ask me how I did it and when I explain them how to do it, I need to tell them to manually remove the generator option from book.json and this adds more friction to the process. As for as I know, there is no good reason to specify the generator option to be ```site``` in book.json, since ```site``` is the default value anyways.