Remove ignored bot_update "force" parameter.

scripts/slave/recipes/chromium_trybot.expected/invalid_results.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/linux",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 487 matching lines @@
       "--revision_mapping_file",
       "{\"src\": \"got_revision\", \"src/buildtools\": \"got_buildtools_revision\", \"src/native_client\": \"got_nacl_revision\", \"src/third_party/angle\": \"got_angle_revision\", \"src/third_party/webrtc\": \"got_webrtc_revision\", \"src/tools/swarming_client\": \"got_swarming_client_revision\", \"src/v8\": \"got_v8_revision\"}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--rietveld_server",
       "https://codereview.chromium.org",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
-      "--force",
       "--output_manifest"
     ],
     "cwd": "[BUILDER_CACHE]/linux",
     "env": {
       "GCLIENT_KILL_GIT_FETCH_AFTER": "1200",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "bot_update (without patch)",
     "~followup_annotations": [
@@ skipping 202 matching lines @@
       "@@@SET_BUILD_PROPERTY@failure_hash@\"212d0eb2385b1eaa828053689225258ece065725\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('base_unittests') failed with return_code 1",
     "recipe_result": null,
     "status_code": 1
   }
 ]