goma: back to build-provided client and disable oauth2

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_linux_memcheck.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "linux_memcheck",
@@ skipping 104 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
-      "--goma-service-account-json-file",
-      "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[BUILD]/goma",
       "--goma-fail-fast",
       "--goma-disable-local-fallback",
       "--",
       "All"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile",
     "~followup_annotations": [
@@ skipping 480 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "webrtc_nonparallel_tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]