cargo.bbclass: replace with what was cargo_util.bbclass

The old cargo.bbclass had no users in meta-rust and had lots of
problems (not least of which was lots of duplicated lines with
cargo_util.bbclass).  Delete the old cargo.bbclass and replace it
entirely wiht cargo_util

recipes-example/rustfmt/rustfmt_0.4.0.bb

@@ -1,4 +1,4 @@
-inherit cargo_util
+inherit cargo
 
 CARGO_INDEX_COMMIT = "3b3994e099281c394a6a66604d1af6c0920e4c31"