Remove ignored bot_update "force" parameter.

scripts/slave/recipes/pdfium.expected/win_xfa.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]\\resources\\bot_update.py",
       "--master",
       "client.pdfium",
       "--builder",
       "windows_xfa",
       "--slave",
       "test_slave",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': '.DEPS.git', 'managed': True, 'name': 'pdfium', 'url': 'https://pdfium.googlesource.com/pdfium.git'}]",
       "--root",
       "pdfium",
       "--revision_mapping_file",
       "{}",
       "--git-cache-dir",
       "[GIT_CACHE]",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "pdfium@HEAD",
-      "--force",
       "--output_manifest"
     ],
     "env": {
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": true, @@@",
@@ skipping 89 matching lines @@
       "out\\Debug"
     ],
     "cwd": "[SLAVE_BUILD]\\pdfium",
     "name": "corpus tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
-]
+]