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

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_win_swarming.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]\\resources\\bot_update.py",
       "--master",
       "tryserver.webrtc",
       "--builder",
       "win_swarming",
@@ skipping 138 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=shared_library dcheck_always_on=1 fastbuild=1 ffmpeg_branding=Chrome gomadir='[CACHE]\\cipd\\goma' rtc_use_h264=1 target_arch=ia32 test_isolation_mode=prepare use_goma=1",
+      "GYP_DEFINES": "component=shared_library dcheck_always_on=1 fastbuild=1 ffmpeg_branding=Chrome gomadir='[CACHE]\\cipd\\goma' rtc_use_h264=1 target_arch=ia32 test_isolation_mode=prepare use_goma=1",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]\\src\\tools\\swarming_client\\swarming.py",
       "--version"
@@ skipping 2214 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]