goma: back to build-provided client and disable oauth2

scripts/slave/recipes/webrtc/ios.expected/basic.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "host and tools info",
     "~followup_annotations": [
@@ skipping 167 matching lines @@
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma-clang",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
-      "--goma-service-account-json-file",
-      "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[BUILD]/goma",
       "--",
       "All"
     ],
     "cwd": "[SLAVE_BUILD]/src/out/Debug-iphonesimulator",
     "name": "compile",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
@@ skipping 68 matching lines @@
       "@@@STEP_LOG_END@fake log@@@",
       "@@@STEP_LINK@fake URL text@fake URL@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]