Remove ignored "force" parameter from bot_update.

infra/recipes/blink_downstream.expected/webkit_tests_unexpected_error.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]/V8_Blink_Linux_64",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 28 matching lines @@
       "--revision_mapping_file",
       "{\"src\": \"got_cr_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_revision\"}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--revision",
       "src/v8@20123",
-      "--force",
       "--output_manifest"
     ],
     "cwd": "[BUILDER_CACHE]/V8_Blink_Linux_64",
     "env": {
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
@@ skipping 322 matching lines @@
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('webkit_tests') failed with return_code 1",
     "recipe_result": null,
     "status_code": 1
   }
 ]