Headless: Remove obsolete Mojo code and some other obsolete APIs

headless/lib/headless_devtools_client_browsertest.cc

Index: headless/lib/headless_devtools_client_browsertest.cc
diff --git a/headless/lib/headless_devtools_client_browsertest.cc b/headless/lib/headless_devtools_client_browsertest.cc
index 79c5495da9ff7b9727d339df63f7d8744f9f36f7..21028f28740e67fbd8c9ed9f9a65d885f8921798 100644
--- a/headless/lib/headless_devtools_client_browsertest.cc
+++ b/headless/lib/headless_devtools_client_browsertest.cc
@@ -1011,10 +1011,6 @@ class RawDevtoolsProtocolTest
         "\"value\":2,\"description\":\"2\"}}}",
         json_message);
 
-    int frame_tree_node_id = 0;
-    EXPECT_TRUE(web_contents_->GetFrameTreeNodeIdForDevToolsAgentHostId(
-        devtools_agent_host_id, &frame_tree_node_id));
-    EXPECT_NE(0, frame_tree_node_id);
     FinishAsynchronousTest();
     return true;
   }