Remove ignored bot_update "force" parameter.

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Win10_Tester.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 34 matching lines @@
       "--root",
       "src",
       "--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/libjingle/source/talk\": \"got_libjingle_revision\", \"src/third_party/libvpx/source\": \"got_libvpx_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]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@1111111",
-      "--force",
       "--output_manifest"
     ],
     "cwd": "[BUILDER_CACHE]\\Win10_Tester",
     "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 421 matching lines @@
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
     },
     "name": "process_dumps"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]