remove comment regarding a removed test directive

Author: tshepang

src/bootstrap/src/core/build_steps/test.rs

@@ -1908,8 +1908,6 @@ NOTE: if you're sure you want to do this, please open an issue as to why. In the
                 llvm_components_passed = true;
             }
             if !builder.is_rust_llvm(target) {
-                // FIXME: missing Rust patches is not the same as being system llvm; we should rename the flag at some point.
-                // Inspecting the tests with `// no-system-llvm` in src/test *looks* like this is doing the right thing, though.
                 cmd.arg("--system-llvm");
             }