remove outdated FIXME

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/core/build_steps/doc.rs

@@ -65,8 +65,6 @@ macro_rules! book {
 // NOTE: When adding a book here, make sure to ALSO build the book by
 // adding a build step in `src/bootstrap/code/builder/mod.rs`!
 // NOTE: Make sure to add the corresponding submodule when adding a new book.
-// FIXME: Make checking for a submodule automatic somehow (maybe by having a list of all submodules
-// and checking against it?).
 book!(
     CargoBook, "src/tools/cargo/src/doc", "cargo", &[];
     ClippyBook, "src/tools/clippy/book", "clippy", &[];