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

scripts/slave/recipes/dart/dartium_build.expected/basic_win_64_MirrorFalse.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "config",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'dartium.deps', 'url': 'https://dart.googlecode.com/svn/branches/bleeding_edge/deps/dartium.deps'}]"
     ],
@@ skipping 63 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]\\src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json",
-      "GYP_DEFINES": "chromium_win_pch=0 component=static_library fastbuild=1 gomadir='[BUILD]\\goma' linux_strip_binary=1 target_arch=x64 use_goma=1",
+      "GYP_DEFINES": "component=static_library fastbuild=1 gomadir='[BUILD]\\goma' linux_strip_binary=1 target_arch=x64 use_goma=1",
       "GYP_GENERATORS": "ninja",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\tools\\runit.py",
@@ skipping 39 matching lines @@
       "12345"
     ],
     "name": "archive_build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]