Remove out-of-date comment.

This comment -- "by default we ignore everything in the repository" --
was added in rust-lang#65939 when rustfmt was first being introduced for this
repository and (briefly) every directory was ignored. Since then lots of
directories have opted in to formatting, so it is no longer true.

rustfmt.toml

@@ -3,7 +3,6 @@ version = "Two"
 use_small_heuristics = "Max"
 merge_derives = false
 
-# by default we ignore everything in the repository
 # tidy only checks files which are not ignored, each entry follows gitignore style
 ignore = [
     "/build/",