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

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Win7_Tester.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 203 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]\\Win7_Tester\\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 ffmpeg_branding=Chrome gomadir='[CACHE]\\cipd\\goma' proprietary_codecs=1 target_arch=ia32 use_goma=1",
+      "GYP_DEFINES": "component=static_library fastbuild=1 ffmpeg_branding=Chrome gomadir='[CACHE]\\cipd\\goma' proprietary_codecs=1 target_arch=ia32 use_goma=1",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILDER_CACHE]\\Win7_Tester\\src\\testing\\scripts\\get_compile_targets.py",
       "--output",
@@ skipping 255 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
   }
 ]