V8: Remove infra-side gn defaults

scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___debug_builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.v8",
       "--builder",
       "V8 Linux - debug builder",
@@ skipping 137 matching lines @@
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gn.py",
       "--root=[SLAVE_BUILD]/v8",
       "gen",
       "[TMP_BASE]/v8_gn_tmp_1",
-      "--args=use_sysroot=false icu_use_data_file = false v8_test_isolation_mode = \"prepare\" is_debug=true target_cpu=\"x86\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "--args=is_debug=true target_cpu=\"x86\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
     ],
     "name": "gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/v8/tools/gyp_flag_compare.py",
       "[TMP_BASE]/v8_gn_tmp_1",
       "[SLAVE_BUILD]/v8/out/Debug",
@@ skipping 266 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@V8 Linux - debug<br />V8 Linux - debug - avx2<br />V8 Linux - debug - greedy allocator<br />V8 Linux - gc stress@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]