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/rust-hello-world/rust-hello-world_git.bb

@@ -1,4 +1,4 @@
-inherit cargo_util
+inherit cargo
 
 SRC_URI = "git://github.com/jmesmon/rust-hello-world.git;protocol=https"
 SRCREV="e0fa23f1a3cb1eb1407165bd2fc36d2f6e6ad728"