[recipes] Remove build environment vars from default_env

infra/bots/recipes/swarm_test.expected/Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Coverage-Trybot.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/skia/infra/bots/assets/skp/VERSION",
       "/path/to/tmp/"
     ],
     "name": "Get downloaded SKP VERSION"
@@ skipping 330 matching lines @@
       "serialize-8888",
       "gm",
       "_",
       "image-cacherator-from-ctable",
       "--outResultsFile",
       "[SLAVE_BUILD]/out/coverage_results/abc123.cov"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "env": {
       "BUILDTYPE": "Coverage",
-      "CC": "/usr/bin/clang",
       "CHROME_HEADLESS": "1",
-      "CXX": "/usr/bin/clang++",
-      "GYP_DEFINES": "skia_arch_type=x86_64 skia_clang_build=1 skia_gpu=0 skia_warnings_as_errors=0",
       "SKIA_OUT": "[SLAVE_BUILD]/out"
     },
     "name": "dm"
   },
   {
     "cmd": [
       "python",
       "[SLAVE_BUILD]/skia/tools/parse_llvm_coverage.py",
       "--report",
       "[SLAVE_BUILD]/out/coverage_results/abc123.cov",
@@ skipping 26 matching lines @@
       "build_number",
       "5",
       "issue",
       "500",
       "patchset",
       "1"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "env": {
       "BUILDTYPE": "Coverage",
-      "CC": "/usr/bin/clang",
       "CHROME_HEADLESS": "1",
-      "CXX": "/usr/bin/clang++",
-      "GYP_DEFINES": "skia_arch_type=x86_64 skia_clang_build=1 skia_gpu=0 skia_warnings_as_errors=0",
       "SKIA_OUT": "[SLAVE_BUILD]/out"
     },
     "name": "Generate Coverage Data"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport json, os, sys\nif os.path.exists(sys.argv[1]) and os.path.isdir(sys.argv[1]):\n  with open(sys.argv[2], 'w') as f:\n    json.dump(os.listdir(sys.argv[1]), f)\n",
       "[SLAVE_BUILD]/out/coverage_results",
@@ skipping 33 matching lines @@
       "[CUSTOM_[SWARM_OUT_DIR]]"
     ],
     "name": "Copy to swarming out (2)"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]