[DevTools] Cleanup DevToolsTarget and DevToolsManagerDelegate after moving to devtools_discovery.

components/devtools_http_handler/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("devtools_http_handler") {
   sources = [
     "devtools_http_handler.cc",
     "devtools_http_handler.h",
     "devtools_http_handler_delegate.h",
   ]
 
   deps = [
     "//base",
+    "//components/devtools_discovery",
     "//content/public/browser",
     "//net",
     "//net:http_server",
   ]
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
     "devtools_http_handler_unittest.cc",
   ]
 
   deps = [
     ":devtools_http_handler",
     "//testing/gtest",
   ]
 }