[recipes] Remove build environment vars from default_env

infra/bots/recipes/swarm_compile.expected/Build-Mac-Clang-Arm7-Release-iOS.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[CUSTOM_/_B_WORK]",
       "511"
     ],
     "name": "makedirs checkout_path",
@@ skipping 15 matching lines @@
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "config",
       "--spec",
       "cache_dir = '[CUSTOM_/_B_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': False, 'name': 'skia', 'url': 'https://skia.googlesource.com/skia.git'}]\ntarget_os = ['llvm']"
     ],
     "cwd": "[CUSTOM_/_B_WORK]",
     "env": {
       "BUILDTYPE": "Release",
-      "CC": "/usr/bin/clang",
       "CHROME_HEADLESS": "1",
-      "CXX": "/usr/bin/clang++",
-      "GYP_DEFINES": "skia_arch_type=arm skia_clang_build=1 skia_os=ios skia_warnings_as_errors=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_/_B_WORK]/skia/out/Build-Mac-Clang-Arm7-Release-iOS"
     },
     "name": "gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "sync",
       "--nohooks",
       "--force",
       "--verbose",
       "--delete_unversioned_trees",
       "--revision",
       "skia@abc123",
       "--output-json",
       "/path/to/tmp/json"
     ],
     "cwd": "[CUSTOM_/_B_WORK]",
     "env": {
       "BUILDTYPE": "Release",
-      "CC": "/usr/bin/clang",
       "CHROME_HEADLESS": "1",
-      "CXX": "/usr/bin/clang++",
-      "GYP_DEFINES": "skia_arch_type=arm skia_clang_build=1 skia_os=ios skia_warnings_as_errors=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]:RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_/_B_WORK]/skia/out/Build-Mac-Clang-Arm7-Release-iOS"
     },
     "name": "gclient sync",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"solutions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"skia/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 164710@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
@@ skipping 92 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    shutil.move(f, dst_path)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]