Remove use_allocator=none where it's implied.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.webkit",
       "--builder",
       "WebKit Linux ASAN",
@@ skipping 65 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "asan=1 clang=1 component=static_library gomadir='[BUILD]/goma' lsan=1 target_arch=x64 test_isolation_mode=prepare use_allocator=none use_goma=1"
+      "GYP_DEFINES": "asan=1 clang=1 component=static_library gomadir='[BUILD]/goma' lsan=1 target_arch=x64 test_isolation_mode=prepare use_goma=1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/src/testing/buildbot/chromium.webkit.json",
       "/path/to/tmp/json"
@@ skipping 227 matching lines @@
     "name": "[collect error] browser_tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys@@@",
       "@@@STEP_LOG_LINE@python.inline@print '%s wasn\\'t triggered' % sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@sys.exit(1)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   }
 ]