Remove additional_target_cpus = [ "arm" ] from iOS device build configs

The configs committed in https://codereview.webrtc.org/2919783002/
was not passing GN and we're still unable to use trybots to test them
due to crbug.com/725910.

BUG=chromium:704912
NOTRY=True
TBR=smut@google.com

Review-Url: https://codereview.webrtc.org/2919693004
Cr-Commit-Position: refs/heads/master@{#18398}

tools_webrtc/ios/internal.client.webrtc/iOS64_Debug.json

@@ -5,7 +5,6 @@
   "xcode version": "8.0",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
-    "additional_target_cpus=[\"arm\"]",
     "is_component_build=false",
     "is_debug=true",
     "target_cpu=\"arm64\"",

tools_webrtc/ios/internal.tryserver.webrtc/ios_arm64.json

@@ -5,7 +5,6 @@
   "xcode version": "8.0",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
-    "additional_target_cpus=[\"arm\"]",
     "is_component_build=false",
     "is_debug=true",
     "target_cpu=\"arm64\"",