compiletest: remove useless path join in rustdoc_json runtest logic

Author: jieyouxu

src/tools/compiletest/src/runtest/rustdoc_json.rs

@@ -16,13 +16,12 @@ impl TestCx<'_> {
             self.fatal_proc_rec("rustdoc failed!", &proc_res);
         }
 
-        let root = self.config.find_rust_src_root().unwrap();
         let mut json_out = out_dir.join(self.testpaths.file.file_stem().unwrap());
         json_out.set_extension("json");
         let res = self.run_command_to_procres(
             Command::new(self.config.jsondocck_path.as_ref().unwrap())
                 .arg("--doc-dir")
-                .arg(root.join(&out_dir))
+                .arg(&out_dir)
                 .arg("--template")
                 .arg(&self.testpaths.file),
         );