iOS: Cleanup buildbot JSON files + bump iOS version to 10.0

webrtc/build/ios/client.webrtc/iOS32_Simulator_Debug.json

 {
   "owners": [
     "kjellander"
   ],
   "comments": [
-    "Tests for 32-bit iOS 9.0 simulators.",
-    "GYP_DEFINES and gn_args in this file are only used during runhooks; ",
-    "webrtc/build/mb_config.pyl decides the generated projects that are compiled."
+    "Tests for 32-bit iOS simulators.",
+    "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",
-  "GYP_DEFINES": [
-    "use_goma=1",
-    "gomadir=$(goma_dir)",
-    "chromium_ios_signing=0",
-    "component=static_library",
-    "target_arch=ia32"
+  "gn_args": [
+    "use_goma=true"
   ],
-  "gn_args": [
-    "is_debug=true",
-    "target_os=\"ios\"",
-    "ios_enable_code_signing=false",
-    "is_component_build=false",
-    "use_goma=true",
-    "goma_dir=\"$(goma_dir)\"",
-    "target_cpu=\"x86\""
-  ],
-  "mb_type": "gn",
   "compiler": "ninja",
   "additional_compile_targets": [ "all" ],
   "configuration": "Debug",
-  "sdk": "iphonesimulator9.0",
+  "sdk": "iphonesimulator10.0",
   "tests": [
     {
       "include": "common_tests.json",
-      "device type": "iPhone 5s",
-      "os": "9.0"
+      "device type": "iPhone 6s",
+      "os": "10.0"
     }
   ]
 }