remove text-direction from book.toml

setting the language to "fa" should make the book rtl

Author: szabgab

book.toml

@@ -5,7 +5,6 @@
 title = "The Rust Programming Language"
 authors = ["Steve Klabnik", "Carol Nichols", "Chris Krycho", "Contributions from the Rust Community"]
 language = "fa"
-text-direction = "rtl"
 
 [output.html]
 additional-css = ["ferris.css", "theme/2018-edition.css", "theme/semantic-notes.css", "theme/listing.css", "theme/custom.css"]