Remove book.multilingual

[nibrobb]

The key book.multilingual was recently removed (rust-lang/mdBook#2775) and the book does not build any more. After removing the key, another error popped up:

Unable to parse the input
 WARN Error writing the RenderContext to the backend, Broken pipe (os error 32)
ERROR The "exercise-linker" preprocessor exited unsuccessfully with exit status: 1 status
Error: Process completed with exit code 101.