remove unused plugin config

Author: joshrotenberg

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

@@ -34,8 +34,3 @@ use-boolean-and = true
 [output.html.fold]
 enable = true
 level = 3
-
-[preprocessor.toc]
-command = "mdbook-toc"
-renderer = ["html"]
-max-level = 3