Remove book.json file

[janodvarko]

Should we remove the book.json if it has no effect?
Added in 05bf08c

Honza

[janodvarko]

@captainbrosset WDYT?

[captainbrosset]

Yes we should. This was a first attempt at fixing the issue we had with the gitbook build failure, but this was not the right place to have this file. Let's remove it.