Remove clang_xcode=0 from iOS ninja device bots

BUG=440789

Review URL: https://codereview.chromium.org/950903006

Cr-Commit-Position: refs/heads/master@{#318258}

ios/build/bots/chromium.fyi/Chromium_iOS_Device_(ninja).json

@@ -8,7 +8,6 @@
   "xcode version": "5.1.1",
   "GYP_DEFINES": {
     "chromium_ios_signing": "0",
-    "clang_xcode": "0",
     "target_subarch": "both"
   },
   "compiler": "ninja",

ios/build/bots/chromium.mac/iOS_Device_(ninja).json

@@ -8,7 +8,6 @@
   "xcode version": "5.1.1",
   "GYP_DEFINES": {
     "chromium_ios_signing": "0",
-    "clang_xcode": "0",
     "target_subarch": "both"
   },
   "compiler": "ninja",

ios/build/bots/tryserver.chromium.mac/ios_rel_device_ninja.json

@@ -8,7 +8,6 @@
   "xcode version": "5.1",
   "GYP_DEFINES": {
     "chromium_ios_signing": "0",
-    "clang_xcode": "0",
     "target_subarch": "both"
   },
   "compiler": "ninja",