recipes/win: Stop explicitly disabling precompiled headers in recipes in goma builds.

scripts/slave/recipes/v8.expected/full_client_v8_V8_Win32___debug_builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 132 matching lines @@
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]\\v8",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json",
       "GYP_CHROMIUM_NO_ACTION": "1",
-      "GYP_DEFINES": "chromium_win_pch=0 fastbuild=1 gomadir='[CACHE]\\cipd\\goma' target_arch=ia32 test_isolation_mode=prepare use_goma=1 v8_enable_slow_dchecks=1 v8_optimized_debug=1 v8_target_arch=ia32",
+      "GYP_DEFINES": "fastbuild=1 gomadir='[CACHE]\\cipd\\goma' target_arch=ia32 test_isolation_mode=prepare use_goma=1 v8_enable_slow_dchecks=1 v8_optimized_debug=1 v8_target_arch=ia32",
       "GYP_GENERATORS": "ninja",
       "GYP_MSVS_VERSION": "2013",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 245 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@V8 Win32 - debug@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]