Fix 'unknown-media-type' compile-fail test.

codegen/tests/compile-fail/unknown-media-type.rs

@@ -1,3 +1,5 @@
+// must-compile-successfully
+
 #![feature(plugin)]
 #![plugin(rocket_codegen)]
 
@@ -12,7 +14,4 @@ fn two() -> &'static str { "hi" }
 #[get("/", format = "x-custom/x-custom")] //~ WARNING not a known media type
 fn three() -> &'static str { "hi" }
 
-// Make the test fail here so we can actually check for the warnings above.
-assert!(false);
-
 fn main() {  }