Merge pull request rust-lang#21064 from lnicola/book-multilingual

minor: Drop `multilingual` field from the book config

Author: lnicola

src/tools/rust-analyzer/docs/book/book.toml

@@ -1,7 +1,6 @@
 [book]
 authors = ["The rust-analyzer authors"]
 language = "en"
-multilingual = false
 src = "src"
 title = "rust-analyzer"