libyuv: Remove all GYP builders.

scripts/slave/recipes/libyuv/libyuv.expected/tryserver_libyuv_ios_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/cleanup_temp.py"
     ],
@@ skipping 118 matching lines @@
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_CROSSCOMPILE": "1",
-      "GYP_DEFINES": "OS=ios clang=1 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[CACHE]/cipd/goma' target_arch=arm use_goma=1",
+      "GYP_DEFINES": "clang=1 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[CACHE]/cipd/goma' target_arch=arm use_goma=1",
       "GYP_GENERATORS": "ninja",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
@@ skipping 73 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]