Revert of iOS: Use JSON for GN configuration instead of MB + remove symbols

tools-webrtc/ios/client.webrtc/iOS64_Debug.json

 {
   "comments": [
-    "Builder for 64-bit devices."
+    "Builder for 64-bit devices.",
+    "gn_args in this file is only read by the buildbot recipe; ",
+    "webrtc/build/mb_config.pyl contains the actual build configuration.",
+    "TODO(kjellander): remove gn_args when the recipe is updated."
   ],
   "xcode version": "8.0",
   "gn_args": [
-    "goma_dir=\"$(goma_dir)\"",
-    "ios_enable_code_signing=false",
-    "is_component_build=false",
     "is_debug=true",
     "target_cpu=\"arm64\"",
-    "target_os=\"ios\"",
     "use_goma=true"
   ],
   "additional_compile_targets": [ "all" ],
   "configuration": "Debug",
   "sdk": "iphoneos10.0",
   "tests": [
   ]
 }