Skip serializing of the multilingual field

This skips serializing of the multilingual field since it is unused, and
we plan to remove it in the future. This helps avoid it showing up in
`mdbook init`.

src/config.rs

@@ -421,6 +421,9 @@ pub struct BookConfig {
     /// Location of the book source relative to the book's root directory.
     pub src: PathBuf,
     /// Does this book support more than one language?
+    // TODO: Remove this field in 0.5, it is unused:
+    // https://github.com/rust-lang/mdBook/issues/2636
+    #[serde(skip_serializing)]
     pub multilingual: bool,
     /// The main language of the book.
     pub language: Option<String>,

tests/testsuite/init.rs

@@ -28,7 +28,6 @@ All done, no errors...
 [book]
 authors = []
 language = "en"
-multilingual = false
 src = "src"
 
 "#]],
@@ -95,7 +94,6 @@ All done, no errors...
 [book]
 authors = []
 language = "en"
-multilingual = false
 src = "src"
 
 "#]],
@@ -129,7 +127,6 @@ All done, no errors...
 [book]
 authors = []
 language = "en"
-multilingual = false
 src = "src"
 title = "Example title"
 
@@ -182,7 +179,6 @@ fn init_with_custom_book_and_src_locations() {
 [book]
 authors = []
 language = "en"
-multilingual = false
 src = "in"
 
 [build]

tests/testsuite/renderer.rs

@@ -188,7 +188,6 @@ fn backends_receive_render_context_via_stdin() {
     "book": {
       "authors": [],
       "language": "en",
-      "multilingual": false,
       "src": "src"
     },
     "output": {