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

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Win_x64_GN.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 217 matching lines @@
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]\\Win_x64_GN\\src",
     "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 component=static_library fastbuild=1 gomadir='[CACHE]\\cipd\\goma' target_arch=x64 test_isolation_mode=prepare use_goma=1",
+      "GYP_DEFINES": "component=static_library fastbuild=1 gomadir='[CACHE]\\cipd\\goma' target_arch=x64 test_isolation_mode=prepare use_goma=1",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILDER_CACHE]\\Win_x64_GN\\src\\testing\\scripts\\get_compile_targets.py",
       "--output",
@@ skipping 130 matching lines @@
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]\\\\build_data_dir\", \"buildername\": \"Win x64 GN\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_buildername\": null, \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]