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-devtools/rust/libstd-rs.bb

@@ -16,7 +16,7 @@ SRC_URI += "\
 DEPENDS += "compiler-rt"
 
 RUSTLIB_DEP = ""
-inherit cargo_util
+inherit cargo
 
 # Needed so cargo can find libbacktrace
 RUSTFLAGS += "-L ${STAGING_LIBDIR}"