rustc_target: Further simplify loading of built-in targets

using the fact that it is infallible.

JSON roundtrip check on every rustc run is also removed, it's already performed by unit tests.

compiler/rustc_driver/src/lib.rs

@@ -672,7 +672,8 @@ impl RustcDefaultCalls {
         for req in &sess.opts.prints {
             match *req {
                 TargetList => {
-                    let mut targets = rustc_target::spec::get_targets().collect::<Vec<String>>();
+                    let mut targets =
+                        rustc_target::spec::TARGETS.iter().copied().collect::<Vec<_>>();
                     targets.sort();
                     println!("{}", targets.join("\n"));
                 }